Is it possible to use *cached* addresses as device's output buffer? E.g, is it a correct thing, to read an uImage from USB pen to cacheable addresses in order to boot from these cached area?

U-Boot 2014.10-00051-g8cb056b-dirty / SDK (May 15 2018 - 17:11:35)

CPU: MIPS32 P5600 @ 1200 MHz (Rev 1.0) FPU: Present Cores: 2 (running on CPU0) Timer: 600 MHz ECC: L1 L2 (80800ff0) PLLs: CPU: 1200MHz SATA: 600MHz ETH:1250MHz PCIE:1200MHz DDR3: 400MHz AXI: 600MHz Board: Baikal-T1 BFK3 Watchdog enabled I2C: ready DRAM: Rank = 1 highmem = 1792 MiB lowmem = 128 MiB NVRAM: ready Rev: 3.1 MIPS: SIMD ready MIPS: Write Merge enable MIPS: MAAR[0]: 0x00010000-0x07ffffff speculate MIPS: secondary cache 1024kB, 8-way, linesize 32 bytes. In: serial Out: serial Err: serial RomID: 20ba181000002360907301001500102604164781 Net: dwmac.bf05e000, dwmac.bf060000

First, read a file to cacheable adresses:

# usb start; fatload usb 0:1 81000000 uimage.nfs (Re)start USB... USB0: Register 1000140 NbrPorts 1 Starting the controller USB XHCI 1.00 scanning bus 0 for devices... 2 USB Device(s) found scanning usb for storage devices... 1 Storage Device(s) found reading uimage.nfs 4630013 bytes read in 2532 ms (1.7 MiB/s) BAIKAL # md 81000000 81000000: 00000000 ffffffff 00000000 ffffffff ................ 81000010: 00000000 ffffffff 00000000 ffffffff ................ 81000020: 00000000 ffffffff 00000000 ffffffff ................ 81000030: 00000000 ffffffff 00000000 ffffffff ................ 81000040: 00000000 ffffffff 00000000 ffffffff ................ 81000050: 00000000 ffffffff 00000000 ffffffff ................ 81000060: 00000000 ffffffff 00000000 ffffffff ................ 81000070: 00000000 ffffffff 00000000 ffffffff ................ 81000080: 00000000 ffffffff 00000000 ffffffff ................ 81000090: 00000000 ffffffff 00000000 ffffffff ................ 810000a0: 00000000 ffffffff 00000000 ffffffff ................ 810000b0: 00000000 ffffffff 00000000 ffffffff ................ 810000c0: 00000000 ffffffff 00000000 ffffffff ................ 810000d0: 00000000 ffffffff 00000000 ffffffff ................ 810000e0: 00000000 ffffffff 00000000 ffffffff ................ 810000f0: 00000000 ffffffff 00000000 ffffffff ................

Then, to the uncacheable area:

# usb start; fatload usb 0:1 a1000000 uimage.nfs (Re)start USB... USB0: Register 1000140 NbrPorts 1 Starting the controller USB XHCI 1.00 scanning bus 0 for devices... 2 USB Device(s) found scanning usb for storage devices... 1 Storage Device(s) found reading uimage.nfs 4630013 bytes read in 2531 ms (1.7 MiB/s)

BAIKAL # md a1000000 a1000000: 56190527 1697a1a2 0107d65a bda54600 '..V....Z....F.. a1000010: 00001080 d0618780 1644ad13 01020505 ......a...D..... a1000020: 756e694c 2e342d78 31312e34 66622d38 Linux-4.4.118-bf a1000030: 302d336b 33333335 6361672d 66623237 k3-05333-gac72bf a1000040: 00088b1f 00000000 7dec0302 655c7c7d ...........}}|\e a1000050: ce6fef9d eda6499c 69da4f40 3300be99 ..o..I..@O.i...3 a1000060: 691a49ed 880701c0 9261ca32 495d80be .I.i....2.a...]I a1000070: dd04505f 9794931d d5dbdc5d 72c4a2b2 _P......]......r a1000080: 7418a648 5a2c44cc d243745d a6430294 H..t.D,Z]tC...C. a1000090: 0af17960 74540b12 dd5652f5 12baeecb `y....Tt.RV..... a10000a0: 7c50575d 0a5655e9 fbf739b4 9273ce7d ]WP|.UV..9..}.s. a10000b0: 7ad69a69 ee3fdef7 e4f9f247 f39e7333 i..z..?.G...3s.. a10000c0: f7e7bf3c cf7fdefc f1bc88db 1bdfc6f7 <............... a10000d0: 7faa7f7f 454b9f9b f45f9fab c9665b1e ......KE.._..[f. a10000e0: ae9dcaee e3cb8d95 3395f895 653f2f62 ...........3b/?e a10000f0: d6e9dc8e f2f4b9f0 bc069153 d4f75d2f ........S.../]..

and, exactly after loading/reading from uncached addresses:

# md 81000000 81000000: 00000000 ffffffff 00000000 ffffffff ................ 81000010: 00000000 ffffffff 00000000 ffffffff ................ 81000020: 00000000 ffffffff 00000000 ffffffff ................ 81000030: 00000000 ffffffff 00000000 ffffffff ................ 81000040: 00000000 ffffffff 00000000 ffffffff ................ 81000050: 00000000 ffffffff 00000000 ffffffff ................ 81000060: 00000000 ffffffff 00000000 ffffffff ................ 81000070: 00000000 ffffffff 00000000 ffffffff ................ 81000080: 00000000 ffffffff 00000000 ffffffff ................ 81000090: 00000000 ffffffff 00000000 ffffffff ................ 810000a0: 7c50575d 0a5655e9 fbf739b4 9273ce7d ]WP|.UV..9..}.s. 810000b0: 7ad69a69 ee3fdef7 e4f9f247 f39e7333 i..z..?.G...3s.. 810000c0: 00000000 ffffffff 00000000 ffffffff ................ 810000d0: 00000000 ffffffff 00000000 ffffffff ................ 810000e0: 00000000 ffffffff 00000000 ffffffff ................ 810000f0: 00000000 ffffffff 00000000 ffffffff ................

From the config for the board:

#define CONFIG_SYS_SDRAM_BASE 0x80000000 /* Only one bank on lower 512 MB can be reached without MMU */ #define CONFIG_SYS_SDRAM_SIZE 0x08000000

As far, as I can see, usb xhci is prepared to deal with uncached memory buffers:

common/usb_storage.c:

static int usb_stor_BBB_transport(ccb *srb, struct us_data *us) { /* lines were deleted */ ... /* DATA phase + error handling */ result = usb_bulk_msg(us->pusb_dev, pipe, srb->pdata, srb->datalen, &data_actlen, USB_CNTL_TIMEOUT * 5); flush_cache(srb->pdata, srb->datalen); // this is the line I've added; it's not a part of official u-boot tree /* lines were deleted */ }

usb_bulk_msg -> ... -> xhci_bulk_tx (drivers/usb/host/xhci-ring.c) contains the line:

/* flush the buffer before use */ xhci_flush_cache((uint32_t)buffer, length);

xhci_flush_cache is: flush_dcache_range(addr & ~(CACHELINE_SIZE - 1), ALIGN(addr + len, CACHELINE_SIZE));