mmc: dw_mmc: Fix IDMAC operation with pages bigger than 4K

Commit 616f876 ("mmc: pass queue_limits to blk_mq_alloc_disk") [1]
revealed the long living issue in dw_mmc.c driver, existing since the
time when it was first introduced in commit f95f385 ("mmc: dw_mmc:
Add Synopsys DesignWare mmc host driver."), also making kernel boot
broken on platforms using dw_mmc driver with 16K or 64K pages enabled,
with this message in dmesg:

    mmcblk: probe of mmc0:0001 failed with error -22

That's happening because mmc_blk_probe() fails when it calls
blk_validate_limits() consequently, which returns the error due to
failed max_segment_size check in this code:

    /*
     * The maximum segment size has an odd historic 64k default that
     * drivers probably should override.  Just like the I/O size we
     * require drivers to at least handle a full page per segment.
     */
    ...
    if (WARN_ON_ONCE(lim->max_segment_size < PAGE_SIZE))
        return -EINVAL;

In case when IDMAC (Internal DMA Controller) is used, dw_mmc.c always
sets .max_seg_size to 4 KiB:

    mmc->max_seg_size = 0x1000;

The comment in the code above explains why it's incorrect. Arnd
suggested setting .max_seg_size to .max_req_size to fix it, which is
also what some other drivers are doing:

   $ grep -rl 'max_seg_size.*=.*max_req_size' drivers/mmc/host/ | \
     wc -l
   18

This change is not only fixing the boot with 16K/64K pages, but also
leads to a better MMC performance. The linear write performance was
tested on E850-96 board (eMMC only), before commit [1] (where it's
possible to boot with 16K/64K pages without this fix, to be able to do
a comparison). It was tested with this command:

    # dd if=/dev/zero of=somefile bs=1M count=500 oflag=sync

Test results are as follows:

  - 4K pages,  .max_seg_size = 4 KiB:                   94.2 MB/s
  - 4K pages,  .max_seg_size = .max_req_size = 512 KiB: 96.9 MB/s
  - 16K pages, .max_seg_size = 4 KiB:                   126 MB/s
  - 16K pages, .max_seg_size = .max_req_size = 2 MiB:   128 MB/s
  - 64K pages, .max_seg_size = 4 KiB:                   138 MB/s
  - 64K pages, .max_seg_size = .max_req_size = 8 MiB:   138 MB/s

Unfortunately, SD card controller is not enabled in E850-96 yet, so it
wasn't possible for me to run the test on some cheap SD cards to check
this patch's impact on those. But it's possible that this change might
also reduce the writes count, thus improving SD/eMMC longevity.

All credit for the analysis and the suggested solution goes to Arnd.

[1] https://lore.kernel.org/all/[email protected]/

Fixes: f95f385 ("mmc: dw_mmc: Add Synopsys DesignWare mmc host driver.")
Suggested-by: Arnd Bergmann <[email protected]>
Reported-by: Linux Kernel Functional Testing <[email protected]>
Closes: https://lore.kernel.org/all/CA+G9fYtddf2Fd3be+YShHP6CmSDNcn0ptW8qg+stUKW+Cn0rjQ@mail.gmail.com/
Signed-off-by: Sam Protsenko <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Ulf Hansson <[email protected]>

Author: Sam Protsenko

drivers/mmc/host/dw_mmc.c

@@ -2957,8 +2957,8 @@ static int dw_mci_init_slot(struct dw_mci *host)
 	if (host->use_dma == TRANS_MODE_IDMAC) {
 		mmc->max_segs = host->ring_size;
 		mmc->max_blk_size = 65535;
-		mmc->max_seg_size = 0x1000;
-		mmc->max_req_size = mmc->max_seg_size * host->ring_size;
+		mmc->max_req_size = DW_MCI_DESC_DATA_LENGTH * host->ring_size;
+		mmc->max_seg_size = mmc->max_req_size;
 		mmc->max_blk_count = mmc->max_req_size / 512;
 	} else if (host->use_dma == TRANS_MODE_EDMAC) {
 		mmc->max_segs = 64;