[PATCH] mmc: core Drop BLK_BOUNCE_HIGH

[Linus Walleij]

The MMC core sets BLK_BOUNCE_HIGH for devices where dma_mask
is unassigned.

For the majority of MMC hosts this path is never taken: the
OF core will unconditionally assign a 32-bit mask to any
OF device, and most MMC hosts are probed from device tree,
see drivers/of/platform.c:

of_platform_device_create_pdata()
        dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
        if (!dev->dev.dma_mask)
                dev->dev.dma_mask = &dev->dev.coherent_dma_mask;

of_amba_device_create()
        dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
        dev->dev.dma_mask = &dev->dev.coherent_dma_mask;

MMC devices that are probed from ACPI or PCI will likewise
have a proper dma_mask assigned.

The only remaining devices that could have a blank dma_mask
are platform devices instantiated from board files.

These are mostly used on systems without CONFIG_HIGHMEM
enabled which means the block layer will not bounce, and in
the few cases where it is enabled it is not used anyway:
for example some OMAP2 systems such as Nokia n800/n810 will
create a platform_device and not assign a dma_mask, however
they do not have any highmem, so no bouncing will happen
anyway: the block core checks if max_low_pfn >= max_pfn
and this will always be false.

Should it turn out there is a platform_device with blank
DMA mask actually using CONFIG_HIGHMEM somewhere out there
we should set dma_mask for it, not do this trickery.

Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
---
 drivers/mmc/core/queue.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/mmc/core/queue.c b/drivers/mmc/core/queue.c
index a0a2412f62a7..316415588a77 100644
--- a/drivers/mmc/core/queue.c
+++ b/drivers/mmc/core/queue.c
@@ -351,8 +351,6 @@ static void mmc_setup_queue(struct mmc_queue *mq, struct mmc_card *card)
 	if (mmc_can_erase(card))
 		mmc_queue_setup_discard(mq->queue, card);
 
-	if (!mmc_dev(host)->dma_mask || !*mmc_dev(host)->dma_mask)
-		blk_queue_bounce_limit(mq->queue, BLK_BOUNCE_HIGH);
 	blk_queue_max_hw_sectors(mq->queue,
 		min(host->max_blk_count, host->max_req_size / 512));
 	if (host->can_dma_map_merge)

---
base-commit: 6613476e225e090cc9aad49be7fa504e290dd33d
change-id: 20240124-mmc-no-blk-bounce-high-d84e8898c707

Best regards,
-- 
Linus Walleij <linus.walleij@xxxxxxxxxx>