diff --git a/drivers/pci/Makefile b/drivers/pci/Makefile index 6b761b4..62232fc 100644 --- a/drivers/pci/Makefile +++ b/drivers/pci/Makefile @@ -6,15 +6,15 @@ # ifneq ($(CONFIG_DM_PCI),) -obj-$(CONFIG_PCI) += pci-uclass.o +obj-$(CONFIG_PCI) += pci-uclass.o pci_auto.o obj-$(CONFIG_DM_PCI_COMPAT) += pci_compat.o obj-$(CONFIG_PCI_SANDBOX) += pci_sandbox.o obj-$(CONFIG_SANDBOX) += pci-emul-uclass.o obj-$(CONFIG_X86) += pci_x86.o else -obj-$(CONFIG_PCI) += pci.o +obj-$(CONFIG_PCI) += pci.o pci_auto_old.o endif -obj-$(CONFIG_PCI) += pci_auto_common.o pci_auto_old.o pci_common.o pci_rom.o +obj-$(CONFIG_PCI) += pci_auto_common.o pci_common.o pci_rom.o obj-$(CONFIG_FSL_PCI_INIT) += fsl_pci_init.o obj-$(CONFIG_PCI_INDIRECT_BRIDGE) += pci_indirect.o diff --git a/drivers/pci/pci-uclass.c b/drivers/pci/pci-uclass.c index 2828055..792d9cb 100644 --- a/drivers/pci/pci-uclass.c +++ b/drivers/pci/pci-uclass.c @@ -17,6 +17,7 @@ #if defined(CONFIG_X86) && defined(CONFIG_HAVE_FSP) #include #endif +#include "pci_internal.h" DECLARE_GLOBAL_DATA_PTR; @@ -404,7 +405,7 @@ int pci_auto_config_devices(struct udevice *bus) int ret; debug("%s: device %s\n", __func__, dev->name); - ret = pciauto_config_device(hose, dm_pci_get_bdf(dev)); + ret = dm_pciauto_config_device(dev); if (ret < 0) return ret; max_bus = ret; @@ -419,26 +420,16 @@ int pci_auto_config_devices(struct udevice *bus) return sub_bus; } -int dm_pci_hose_probe_bus(struct pci_controller *hose, pci_dev_t bdf) +int dm_pci_hose_probe_bus(struct udevice *bus) { - struct udevice *parent, *bus; int sub_bus; int ret; debug("%s\n", __func__); - parent = hose->bus; - - /* Find the bus within the parent */ - ret = pci_bus_find_devfn(parent, PCI_MASK_BUS(bdf), &bus); - if (ret) { - debug("%s: Cannot find device %x on bus %s: %d\n", __func__, - bdf, parent->name, ret); - return ret; - } sub_bus = pci_get_bus_max() + 1; debug("%s: bus = %d/%s\n", __func__, sub_bus, bus->name); - pciauto_prescan_setup_bridge(hose, bdf, sub_bus); + dm_pciauto_prescan_setup_bridge(bus, sub_bus); ret = device_probe(bus); if (ret) { @@ -452,7 +443,7 @@ int dm_pci_hose_probe_bus(struct pci_controller *hose, pci_dev_t bdf) return -EPIPE; } sub_bus = pci_get_bus_max(); - pciauto_postscan_setup_bridge(hose, bdf, sub_bus); + dm_pciauto_postscan_setup_bridge(bus, sub_bus); return sub_bus; } diff --git a/drivers/pci/pci_auto.c b/drivers/pci/pci_auto.c new file mode 100644 index 0000000..842eafc --- /dev/null +++ b/drivers/pci/pci_auto.c @@ -0,0 +1,386 @@ +/* + * PCI autoconfiguration library + * + * Author: Matt Porter + * + * Copyright 2000 MontaVista Software Inc. + * + * SPDX-License-Identifier: GPL-2.0+ + */ + +#include +#include +#include + +/* the user can define CONFIG_SYS_PCI_CACHE_LINE_SIZE to avoid problems */ +#ifndef CONFIG_SYS_PCI_CACHE_LINE_SIZE +#define CONFIG_SYS_PCI_CACHE_LINE_SIZE 8 +#endif + +void dm_pciauto_setup_device(struct udevice *dev, int bars_num, + struct pci_region *mem, + struct pci_region *prefetch, struct pci_region *io, + bool enum_only) +{ + u32 bar_response; + pci_size_t bar_size; + u16 cmdstat = 0; + int bar, bar_nr = 0; + u8 header_type; + int rom_addr; + pci_addr_t bar_value; + struct pci_region *bar_res; + int found_mem64 = 0; + u16 class; + + dm_pci_read_config16(dev, PCI_COMMAND, &cmdstat); + cmdstat = (cmdstat & ~(PCI_COMMAND_IO | PCI_COMMAND_MEMORY)) | + PCI_COMMAND_MASTER; + + for (bar = PCI_BASE_ADDRESS_0; + bar < PCI_BASE_ADDRESS_0 + (bars_num * 4); bar += 4) { + /* Tickle the BAR and get the response */ + if (!enum_only) + dm_pci_write_config32(dev, bar, 0xffffffff); + dm_pci_read_config32(dev, bar, &bar_response); + + /* If BAR is not implemented go to the next BAR */ + if (!bar_response) + continue; + + found_mem64 = 0; + + /* Check the BAR type and set our address mask */ + if (bar_response & PCI_BASE_ADDRESS_SPACE) { + bar_size = ((~(bar_response & PCI_BASE_ADDRESS_IO_MASK)) + & 0xffff) + 1; + if (!enum_only) + bar_res = io; + + debug("PCI Autoconfig: BAR %d, I/O, size=0x%llx, ", + bar_nr, (unsigned long long)bar_size); + } else { + if ((bar_response & PCI_BASE_ADDRESS_MEM_TYPE_MASK) == + PCI_BASE_ADDRESS_MEM_TYPE_64) { + u32 bar_response_upper; + u64 bar64; + + if (!enum_only) { + dm_pci_write_config32(dev, bar + 4, + 0xffffffff); + } + dm_pci_read_config32(dev, bar + 4, + &bar_response_upper); + + bar64 = ((u64)bar_response_upper << 32) | + bar_response; + + bar_size = ~(bar64 & PCI_BASE_ADDRESS_MEM_MASK) + + 1; + if (!enum_only) + found_mem64 = 1; + } else { + bar_size = (u32)(~(bar_response & + PCI_BASE_ADDRESS_MEM_MASK) + 1); + } + if (!enum_only) { + if (prefetch && (bar_response & + PCI_BASE_ADDRESS_MEM_PREFETCH)) { + bar_res = prefetch; + } else { + bar_res = mem; + } + } + + debug("PCI Autoconfig: BAR %d, %s, size=0x%llx, ", + bar_nr, bar_res == prefetch ? "Prf" : "Mem", + (unsigned long long)bar_size); + } + + if (!enum_only && pciauto_region_allocate(bar_res, bar_size, + &bar_value) == 0) { + /* Write it out and update our limit */ + dm_pci_write_config32(dev, bar, (u32)bar_value); + + if (found_mem64) { + bar += 4; +#ifdef CONFIG_SYS_PCI_64BIT + dm_pci_write_config32(dev, bar, + (u32)(bar_value >> 32)); +#else + /* + * If we are a 64-bit decoder then increment to + * the upper 32 bits of the bar and force it to + * locate in the lower 4GB of memory. + */ + dm_pci_write_config32(dev, bar, 0x00000000); +#endif + } + } + + cmdstat |= (bar_response & PCI_BASE_ADDRESS_SPACE) ? + PCI_COMMAND_IO : PCI_COMMAND_MEMORY; + + debug("\n"); + + bar_nr++; + } + + if (!enum_only) { + /* Configure the expansion ROM address */ + dm_pci_read_config8(dev, PCI_HEADER_TYPE, &header_type); + header_type &= 0x7f; + if (header_type != PCI_HEADER_TYPE_CARDBUS) { + rom_addr = (header_type == PCI_HEADER_TYPE_NORMAL) ? + PCI_ROM_ADDRESS : PCI_ROM_ADDRESS1; + dm_pci_write_config32(dev, rom_addr, 0xfffffffe); + dm_pci_read_config32(dev, rom_addr, &bar_response); + if (bar_response) { + bar_size = -(bar_response & ~1); + debug("PCI Autoconfig: ROM, size=%#x, ", + (unsigned int)bar_size); + if (pciauto_region_allocate(mem, bar_size, + &bar_value) == 0) { + dm_pci_write_config32(dev, rom_addr, + bar_value); + } + cmdstat |= PCI_COMMAND_MEMORY; + debug("\n"); + } + } + } + + /* PCI_COMMAND_IO must be set for VGA device */ + dm_pci_read_config16(dev, PCI_CLASS_DEVICE, &class); + if (class == PCI_CLASS_DISPLAY_VGA) + cmdstat |= PCI_COMMAND_IO; + + dm_pci_write_config16(dev, PCI_COMMAND, cmdstat); + dm_pci_write_config8(dev, PCI_CACHE_LINE_SIZE, + CONFIG_SYS_PCI_CACHE_LINE_SIZE); + dm_pci_write_config8(dev, PCI_LATENCY_TIMER, 0x80); +} + +void dm_pciauto_prescan_setup_bridge(struct udevice *dev, int sub_bus) +{ + struct pci_region *pci_mem; + struct pci_region *pci_prefetch; + struct pci_region *pci_io; + u16 cmdstat, prefechable_64; + /* The root controller has the region information */ + struct pci_controller *ctlr_hose = pci_bus_to_hose(0); + + pci_mem = ctlr_hose->pci_mem; + pci_prefetch = ctlr_hose->pci_prefetch; + pci_io = ctlr_hose->pci_io; + + dm_pci_read_config16(dev, PCI_COMMAND, &cmdstat); + dm_pci_read_config16(dev, PCI_PREF_MEMORY_BASE, &prefechable_64); + prefechable_64 &= PCI_PREF_RANGE_TYPE_MASK; + + /* Configure bus number registers */ + dm_pci_write_config8(dev, PCI_PRIMARY_BUS, + PCI_BUS(dm_pci_get_bdf(dev))); + dm_pci_write_config8(dev, PCI_SECONDARY_BUS, sub_bus); + dm_pci_write_config8(dev, PCI_SUBORDINATE_BUS, 0xff); + + if (pci_mem) { + /* Round memory allocator to 1MB boundary */ + pciauto_region_align(pci_mem, 0x100000); + + /* + * Set up memory and I/O filter limits, assume 32-bit + * I/O space + */ + dm_pci_write_config16(dev, PCI_MEMORY_BASE, + (pci_mem->bus_lower & 0xfff00000) >> 16); + + cmdstat |= PCI_COMMAND_MEMORY; + } + + if (pci_prefetch) { + /* Round memory allocator to 1MB boundary */ + pciauto_region_align(pci_prefetch, 0x100000); + + /* + * Set up memory and I/O filter limits, assume 32-bit + * I/O space + */ + dm_pci_write_config16(dev, PCI_PREF_MEMORY_BASE, + (pci_prefetch->bus_lower & 0xfff00000) >> 16); + if (prefechable_64 == PCI_PREF_RANGE_TYPE_64) +#ifdef CONFIG_SYS_PCI_64BIT + dm_pci_write_config32(dev, PCI_PREF_BASE_UPPER32, + pci_prefetch->bus_lower >> 32); +#else + dm_pci_write_config32(dev, PCI_PREF_BASE_UPPER32, 0x0); +#endif + + cmdstat |= PCI_COMMAND_MEMORY; + } else { + /* We don't support prefetchable memory for now, so disable */ + dm_pci_write_config16(dev, PCI_PREF_MEMORY_BASE, 0x1000); + dm_pci_write_config16(dev, PCI_PREF_MEMORY_LIMIT, 0x0); + if (prefechable_64 == PCI_PREF_RANGE_TYPE_64) { + dm_pci_write_config16(dev, PCI_PREF_BASE_UPPER32, 0x0); + dm_pci_write_config16(dev, PCI_PREF_LIMIT_UPPER32, 0x0); + } + } + + if (pci_io) { + /* Round I/O allocator to 4KB boundary */ + pciauto_region_align(pci_io, 0x1000); + + dm_pci_write_config8(dev, PCI_IO_BASE, + (pci_io->bus_lower & 0x0000f000) >> 8); + dm_pci_write_config16(dev, PCI_IO_BASE_UPPER16, + (pci_io->bus_lower & 0xffff0000) >> 16); + + cmdstat |= PCI_COMMAND_IO; + } + + /* Enable memory and I/O accesses, enable bus master */ + dm_pci_write_config16(dev, PCI_COMMAND, cmdstat | PCI_COMMAND_MASTER); +} + +void dm_pciauto_postscan_setup_bridge(struct udevice *dev, int sub_bus) +{ + struct pci_region *pci_mem; + struct pci_region *pci_prefetch; + struct pci_region *pci_io; + + /* The root controller has the region information */ + struct pci_controller *ctlr_hose = pci_bus_to_hose(0); + + pci_mem = ctlr_hose->pci_mem; + pci_prefetch = ctlr_hose->pci_prefetch; + pci_io = ctlr_hose->pci_io; + + /* Configure bus number registers */ + dm_pci_write_config8(dev, PCI_SUBORDINATE_BUS, sub_bus); + + if (pci_mem) { + /* Round memory allocator to 1MB boundary */ + pciauto_region_align(pci_mem, 0x100000); + + dm_pci_write_config16(dev, PCI_MEMORY_LIMIT, + (pci_mem->bus_lower - 1) >> 16); + } + + if (pci_prefetch) { + u16 prefechable_64; + + dm_pci_read_config16(dev, PCI_PREF_MEMORY_LIMIT, + &prefechable_64); + prefechable_64 &= PCI_PREF_RANGE_TYPE_MASK; + + /* Round memory allocator to 1MB boundary */ + pciauto_region_align(pci_prefetch, 0x100000); + + dm_pci_write_config16(dev, PCI_PREF_MEMORY_LIMIT, + (pci_prefetch->bus_lower - 1) >> 16); + if (prefechable_64 == PCI_PREF_RANGE_TYPE_64) +#ifdef CONFIG_SYS_PCI_64BIT + dm_pci_write_config32(dev, PCI_PREF_LIMIT_UPPER32, + (pci_prefetch->bus_lower - 1) >> 32); +#else + dm_pci_write_config32(dev, PCI_PREF_LIMIT_UPPER32, 0x0); +#endif + } + + if (pci_io) { + /* Round I/O allocator to 4KB boundary */ + pciauto_region_align(pci_io, 0x1000); + + dm_pci_write_config8(dev, PCI_IO_LIMIT, + ((pci_io->bus_lower - 1) & 0x0000f000) >> 8); + dm_pci_write_config16(dev, PCI_IO_LIMIT_UPPER16, + ((pci_io->bus_lower - 1) & 0xffff0000) >> 16); + } +} + +/* + * HJF: Changed this to return int. I think this is required + * to get the correct result when scanning bridges + */ +int dm_pciauto_config_device(struct udevice *dev) +{ + struct pci_region *pci_mem; + struct pci_region *pci_prefetch; + struct pci_region *pci_io; + unsigned int sub_bus = PCI_BUS(dm_pci_get_bdf(dev)); + unsigned short class; + bool enum_only = false; + int n; + +#ifdef CONFIG_PCI_ENUM_ONLY + enum_only = true; +#endif + /* The root controller has the region information */ + struct pci_controller *ctlr_hose = pci_bus_to_hose(0); + + pci_mem = ctlr_hose->pci_mem; + pci_prefetch = ctlr_hose->pci_prefetch; + pci_io = ctlr_hose->pci_io; + + dm_pci_read_config16(dev, PCI_CLASS_DEVICE, &class); + + switch (class) { + case PCI_CLASS_BRIDGE_PCI: + debug("PCI Autoconfig: Found P2P bridge, device %d\n", + PCI_DEV(dm_pci_get_bdf(dev))); + + dm_pciauto_setup_device(dev, 2, pci_mem, pci_prefetch, pci_io, + enum_only); + + n = dm_pci_hose_probe_bus(dev); + if (n < 0) + return n; + sub_bus = (unsigned int)n; + break; + + case PCI_CLASS_BRIDGE_CARDBUS: + /* + * just do a minimal setup of the bridge, + * let the OS take care of the rest + */ + dm_pciauto_setup_device(dev, 0, pci_mem, pci_prefetch, pci_io, + enum_only); + + debug("PCI Autoconfig: Found P2CardBus bridge, device %d\n", + PCI_DEV(dm_pci_get_bdf(dev))); + + break; + +#if defined(CONFIG_PCIAUTO_SKIP_HOST_BRIDGE) + case PCI_CLASS_BRIDGE_OTHER: + debug("PCI Autoconfig: Skipping bridge device %d\n", + PCI_DEV(dm_pci_get_bdf(dev))); + break; +#endif +#if defined(CONFIG_MPC834x) && !defined(CONFIG_VME8349) + case PCI_CLASS_BRIDGE_OTHER: + /* + * The host/PCI bridge 1 seems broken in 8349 - it presents + * itself as 'PCI_CLASS_BRIDGE_OTHER' and appears as an _agent_ + * device claiming resources io/mem/irq.. we only allow for + * the PIMMR window to be allocated (BAR0 - 1MB size) + */ + debug("PCI Autoconfig: Broken bridge found, only minimal config\n"); + dm_pciauto_setup_device(dev, 0, hose->pci_mem, + hose->pci_prefetch, hose->pci_io, + enum_only); + break; +#endif + + case PCI_CLASS_PROCESSOR_POWERPC: /* an agent or end-point */ + debug("PCI AutoConfig: Found PowerPC device\n"); + + default: + dm_pciauto_setup_device(dev, 6, pci_mem, pci_prefetch, pci_io, + enum_only); + break; + } + + return sub_bus; +} diff --git a/drivers/pci/pci_auto_old.c b/drivers/pci/pci_auto_old.c index 1ff88b1..9126f78 100644 --- a/drivers/pci/pci_auto_old.c +++ b/drivers/pci/pci_auto_old.c @@ -180,18 +180,9 @@ void pciauto_prescan_setup_bridge(struct pci_controller *hose, struct pci_region *pci_io; u16 cmdstat, prefechable_64; -#ifdef CONFIG_DM_PCI - /* The root controller has the region information */ - struct pci_controller *ctlr_hose = pci_bus_to_hose(0); - - pci_mem = ctlr_hose->pci_mem; - pci_prefetch = ctlr_hose->pci_prefetch; - pci_io = ctlr_hose->pci_io; -#else pci_mem = hose->pci_mem; pci_prefetch = hose->pci_prefetch; pci_io = hose->pci_io; -#endif pci_hose_read_config_word(hose, dev, PCI_COMMAND, &cmdstat); pci_hose_read_config_word(hose, dev, PCI_PREF_MEMORY_BASE, @@ -199,15 +190,10 @@ void pciauto_prescan_setup_bridge(struct pci_controller *hose, prefechable_64 &= PCI_PREF_RANGE_TYPE_MASK; /* Configure bus number registers */ -#ifdef CONFIG_DM_PCI - pci_hose_write_config_byte(hose, dev, PCI_PRIMARY_BUS, PCI_BUS(dev)); - pci_hose_write_config_byte(hose, dev, PCI_SECONDARY_BUS, sub_bus); -#else pci_hose_write_config_byte(hose, dev, PCI_PRIMARY_BUS, PCI_BUS(dev) - hose->first_busno); pci_hose_write_config_byte(hose, dev, PCI_SECONDARY_BUS, sub_bus - hose->first_busno); -#endif pci_hose_write_config_byte(hose, dev, PCI_SUBORDINATE_BUS, 0xff); if (pci_mem) { @@ -274,26 +260,13 @@ void pciauto_postscan_setup_bridge(struct pci_controller *hose, struct pci_region *pci_prefetch; struct pci_region *pci_io; -#ifdef CONFIG_DM_PCI - /* The root controller has the region information */ - struct pci_controller *ctlr_hose = pci_bus_to_hose(0); - - pci_mem = ctlr_hose->pci_mem; - pci_prefetch = ctlr_hose->pci_prefetch; - pci_io = ctlr_hose->pci_io; -#else pci_mem = hose->pci_mem; pci_prefetch = hose->pci_prefetch; pci_io = hose->pci_io; -#endif /* Configure bus number registers */ -#ifdef CONFIG_DM_PCI - pci_hose_write_config_byte(hose, dev, PCI_SUBORDINATE_BUS, sub_bus); -#else pci_hose_write_config_byte(hose, dev, PCI_SUBORDINATE_BUS, sub_bus - hose->first_busno); -#endif if (pci_mem) { /* Round memory allocator to 1MB boundary */ @@ -353,18 +326,9 @@ int pciauto_config_device(struct pci_controller *hose, pci_dev_t dev) unsigned short class; int n; -#ifdef CONFIG_DM_PCI - /* The root controller has the region information */ - struct pci_controller *ctlr_hose = pci_bus_to_hose(0); - - pci_mem = ctlr_hose->pci_mem; - pci_prefetch = ctlr_hose->pci_prefetch; - pci_io = ctlr_hose->pci_io; -#else pci_mem = hose->pci_mem; pci_prefetch = hose->pci_prefetch; pci_io = hose->pci_io; -#endif pci_hose_read_config_word(hose, dev, PCI_CLASS_DEVICE, &class); @@ -376,12 +340,6 @@ int pciauto_config_device(struct pci_controller *hose, pci_dev_t dev) pciauto_setup_device(hose, dev, 2, pci_mem, pci_prefetch, pci_io); -#ifdef CONFIG_DM_PCI - n = dm_pci_hose_probe_bus(hose, dev); - if (n < 0) - return n; - sub_bus = (unsigned int)n; -#else /* Passing in current_busno allows for sibling P2P bridges */ hose->current_busno++; pciauto_prescan_setup_bridge(hose, dev, hose->current_busno); @@ -396,7 +354,6 @@ int pciauto_config_device(struct pci_controller *hose, pci_dev_t dev) pciauto_postscan_setup_bridge(hose, dev, sub_bus); sub_bus = hose->current_busno; -#endif break; case PCI_CLASS_BRIDGE_CARDBUS: @@ -410,9 +367,7 @@ int pciauto_config_device(struct pci_controller *hose, pci_dev_t dev) debug("PCI Autoconfig: Found P2CardBus bridge, device %d\n", PCI_DEV(dev)); -#ifndef CONFIG_DM_PCI hose->current_busno++; -#endif break; #if defined(CONFIG_PCIAUTO_SKIP_HOST_BRIDGE) diff --git a/drivers/pci/pci_internal.h b/drivers/pci/pci_internal.h new file mode 100644 index 0000000..0867575 --- /dev/null +++ b/drivers/pci/pci_internal.h @@ -0,0 +1,50 @@ +/* + * Internal PCI functions, not exported outside drivers/pci + * + * Copyright (c) 2015 Google, Inc + * Written by Simon Glass + * + * SPDX-License-Identifier: GPL-2.0+ + */ + +#ifndef __pci_internal_h +#define __pci_internal_h + +/** + * dm_pciauto_prescan_setup_bridge() - Set up a bridge for scanning + * + * This gets a bridge ready so that its downstream devices can be scanned. + * It sets up the bus number and memory range registers. Once the scan is + * completed, dm_pciauto_postscan_setup_bridge() should be called. + * + * @dev: Bridge device to be scanned + * @sub_bus: Bus number of the 'other side' of the bridge + */ +void dm_pciauto_prescan_setup_bridge(struct udevice *dev, int sub_bus); + +/** + * dm_pciauto_postscan_setup_bridge() - Finish set up of a bridge after scanning + * + * This should be called after a bus scan is complete. It adjusts the memory + * ranges to fit with the devices actually found on the other side (downstream) + * of the bridge. + * + * @dev: Bridge device that was scanned + * @sub_bus: Bus number of the 'other side' of the bridge + */ +void dm_pciauto_postscan_setup_bridge(struct udevice *dev, int sub_bus); + +/** + * dm_pciauto_config_device() - Configure a PCI device ready for use + * + * If the device is a bridge, downstream devices will be probed. + * + * @dev: Device to configure + * @return the maximum PCI bus number found by this device. If there are no + * bridges, this just returns the device's bus number. If the device is a + * bridge then it will return a larger number, depending on the devices on + * that bridge. On error, returns a -ve error number. + */ +int dm_pciauto_config_device(struct udevice *dev); + +#endif diff --git a/include/pci.h b/include/pci.h index 5861976..182290c 100644 --- a/include/pci.h +++ b/include/pci.h @@ -995,7 +995,7 @@ int pci_find_device_id(struct pci_device_id *ids, int index, * @bdf: PCI bus address to scan (PCI_BUS(bdf) is the bus number) * @return 0 if OK, -ve on error */ -int dm_pci_hose_probe_bus(struct pci_controller *hose, pci_dev_t bdf); +int dm_pci_hose_probe_bus(struct udevice *bus); /** * pci_bus_read_config() - Read a configuration value from a device