Re: [PATCH] mmc: dw_mmc: Fix IDMAC operation with pages bigger than 4K

Ron Economos
2024-09-16T06:10:00Z

On 3/6/24 3:20 PM, Sam Protsenko wrote:
> Commit 616f87661792 ("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 f95f3850f7a9 ("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/20240215070300.2200308-18-hch@lst.de/
>
> Fixes: f95f3850f7a9 ("mmc: dw_mmc: Add Synopsys DesignWare mmc host driver.")
> Suggested-by: Arnd Bergmann <arnd@arndb.de>
> Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
> Closes: https://lore.kernel.org/all/CA+G9fYtddf2Fd3be+YShHP6CmSDNcn0ptW8qg+stUKW+Cn0rjQ@mail.gmail.com/
> Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
> ---
> drivers/mmc/host/dw_mmc.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/mmc/host/dw_mmc.c b/drivers/mmc/host/dw_mmc.c
> index 8e2d676b9239..cccd5633ff40 100644
> --- a/drivers/mmc/host/dw_mmc.c
> +++ b/drivers/mmc/host/dw_mmc.c
> @@ -2951,8 +2951,8 @@ 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;

Unfortunately, this patch creates side effects for the RISC-V StarFive
VisionFive2 board (which uses the DesignWare controller). The following
swiotlb buffer warnings are produced at boot:

2024-09-15T17:54:27.194101-07:00 visionfive kernel: dwmmc_starfive
16020000.mmc: swiotlb buffer is full (sz: 352256 bytes), total 32768
(slots), used 222 (slots)
2024-09-15T17:54:27.194107-07:00 visionfive kernel: dwmmc_starfive
16020000.mmc: swiotlb buffer is full (sz: 352256 bytes), total 32768
(slots), used 222 (slots)
2024-09-15T17:54:27.194123-07:00 visionfive kernel: dwmmc_starfive
16020000.mmc: swiotlb buffer is full (sz: 524288 bytes), total 32768
(slots), used 110 (slots)
2024-09-15T17:54:27.194129-07:00 visionfive kernel: dwmmc_starfive
16020000.mmc: swiotlb buffer is full (sz: 524288 bytes), total 32768
(slots), used 110 (slots)