[PATCH 6/9] mmc: sdhci: allow drivers to pre-allocate bounce buffer

Albert Yang
2025-09-23T06:46:05Z

In sdhci_allocate_bounce_buffer(), add an early path that respects a
driver-provided pre-allocated bounce buffer (host->bounce_buffer).

If the buffer is already allocated by the driver (e.g. coherent/SRAM
buffer needed for platforms with 32-bit DMA constraints), just compute
max_blocks from host->bounce_buffer_size and jump to the common "out"
path to set mmc->max_*.

This enables platform drivers to allocate the bounce buffer before
sdhci_add_host(), avoiding starting the host without the buffer ready
and aligning with the guidance from review.

No functional change for drivers that do not pre-allocate the buffer.

drivers/mmc/host/sdhci.c (sdhci_allocate_bounce_buffer): Handle
pre-allocated bounce buffer and fall through to set mmc->max_*.

Suggested-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/lkml/2b23bcb9-abc7-4667-b939-a19ecae935a2@intel.com/
Signed-off-by: Albert Yang <yangzh0906@thundersoft.com>

---
Changes for v4:
- Add new patch by Suggested-by: Adrian Hunter
---
drivers/mmc/host/sdhci.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/drivers/mmc/host/sdhci.c b/drivers/mmc/host/sdhci.c
index 3a17821efa5ca92c6c29141d8fcb9ebf58355cc7..03fbe30cb205e16b924481caa44f0979d230f380 100644
--- a/drivers/mmc/host/sdhci.c
+++ b/drivers/mmc/host/sdhci.c
@@ -4193,6 +4193,12 @@ static void sdhci_allocate_bounce_buffer(struct sdhci_host *host)
unsigned int bounce_size;
int ret;

+ /* Drivers may have already allocated the buffer */
+ if (host->bounce_buffer) {
+ bounce_size = host->bounce_buffer_size;
+ max_blocks = bounce_size / 512;
+ goto out;
+ }
/*
* Cap the bounce buffer at 64KB. Using a bigger bounce buffer
* has diminishing returns, this is probably because SD/MMC
@@ -4241,6 +4247,7 @@ static void sdhci_allocate_bounce_buffer(struct sdhci_host *host)

host->bounce_buffer_size = bounce_size;

+out:
/* Lie about this since we're bouncing */
mmc->max_segs = max_blocks;
mmc->max_seg_size = bounce_size;

--
2.43.0