[PATCH] dmaengine: stm32-dma: avoid bitfield overflow assertion

[Arnd Bergmann]

From: Arnd Bergmann <arnd@xxxxxxxx>

stm32_dma_get_burst() returns a negative error code for invalid
input, which gets turned into a large u32 value in stm32_dma_prep_dma_memcpy()
that in turn triggers an assertion because it does not fit into a
two-bit field:

drivers/dma/stm32-dma.c: In function 'stm32_dma_prep_dma_memcpy':
include/linux/compiler_types.h:399:38: error: call to '__compiletime_assert_310' declared with attribute error: FIELD_PREP: value too large for the field
  399 |  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |                                      ^
include/linux/compiler_types.h:380:4: note: in definition of macro '__compiletime_assert'
  380 |    prefix ## suffix();    \
      |    ^~~~~~
include/linux/compiler_types.h:399:2: note: in expansion of macro '_compiletime_assert'
  399 |  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |  ^~~~~~~~~~~~~~~~~~~
include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
   39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
      |                                     ^~~~~~~~~~~~~~~~~~
include/linux/bitfield.h:68:3: note: in expansion of macro 'BUILD_BUG_ON_MSG'
   68 |   BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?  \
      |   ^~~~~~~~~~~~~~~~
include/linux/bitfield.h:114:3: note: in expansion of macro '__BF_FIELD_CHECK'
  114 |   __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: "); \
      |   ^~~~~~~~~~~~~~~~
drivers/dma/stm32-dma.c:1273:4: note: in expansion of macro 'FIELD_PREP'
 1273 |    FIELD_PREP(STM32_DMA_SCR_PBURST_MASK, dma_burst) |
      |    ^~~~~~~~~~

I only see this with older gcc versions like gcc-6.5 or gcc-9.5 but not
with gcc-12.2 or higher. My best guess is that this is the result of
changes to __builtin_constant_p(), which seems to treat the 'cold'
codepath after an error message as a constant branch, while in newer
gcc versions the range check is skipped after determining that
dma_burst is never a compile-time constant.

As an easy workaround, assume the error can happen, so try to handle this
by failing stm32_dma_prep_dma_memcpy() before the assertion.

Fixes: 1c32d6c37cc2 ("dmaengine: stm32-dma: use bitfield helpers")
Fixes: a2b6103b7a8a ("dmaengine: stm32-dma: Improve memory burst management")
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
---
 drivers/dma/stm32-dma.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/dma/stm32-dma.c b/drivers/dma/stm32-dma.c
index 37674029cb42..e3cd4b0525e6 100644
--- a/drivers/dma/stm32-dma.c
+++ b/drivers/dma/stm32-dma.c
@@ -1266,6 +1266,10 @@ static struct dma_async_tx_descriptor *stm32_dma_prep_dma_memcpy(
 		best_burst = stm32_dma_get_best_burst(len, STM32_DMA_MAX_BURST,
 						      threshold, max_width);
 		dma_burst = stm32_dma_get_burst(chan, best_burst);
+		if (dma_burst > 3) {
+			kfree(desc);
+			return NULL;
+		}
 
 		stm32_dma_clear_reg(&desc->sg_req[i].chan_reg);
 		desc->sg_req[i].chan_reg.dma_scr =
-- 
2.39.1