linux-kernel - [PATCH 1/1] dmaengine: stm32-dma: avoid 64-bit division in stm32_dma_get_max

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Message-ID: <20211103153312.41483-1-amelie.delaunay@foss.st.com>
Date:   Wed, 3 Nov 2021 16:33:12 +0100
From:   Amelie Delaunay <amelie.delaunay@...s.st.com>
To:     Vinod Koul <vkoul@...nel.org>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>
CC:     <dmaengine@...r.kernel.org>,
        <linux-stm32@...md-mailman.stormreply.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>,
        Amelie Delaunay <amelie.delaunay@...s.st.com>,
        Arnd Bergmann <arnd@...db.de>,
        kernel test robot <lkp@...el.com>
Subject: [PATCH 1/1] dmaengine: stm32-dma: avoid 64-bit division in stm32_dma_get_max_width

From: Arnd Bergmann <arnd@...db.de>

Using the % operator on a 64-bit variable is expensive and can
cause a link failure:

arm-linux-gnueabi-ld: drivers/dma/stm32-dma.o: in function `stm32_dma_get_max_width':
stm32-dma.c:(.text+0x170): undefined reference to `__aeabi_uldivmod'
arm-linux-gnueabi-ld: drivers/dma/stm32-dma.o: in function `stm32_dma_set_xfer_param':
stm32-dma.c:(.text+0x1cd4): undefined reference to `__aeabi_uldivmod'

As we know that we just want to check the alignment in
stm32_dma_get_max_width(), there is no need for a full division, and
using a simple mask is a faster replacement.

Same in stm32_dma_set_xfer_param(), change this to only allow burst
transfers if the address is a multiple of the length.
stm32_dma_get_best_burst just after will take buf_len into account to fix
burst in case of misalignment.

Fixes: b20fd5fa310c ("dmaengine: stm32-dma: fix stm32_dma_get_max_width")
Reported-by: kernel test robot <lkp@...el.com>
Signed-off-by: Arnd Bergmann <arnd@...db.de>
Signed-off-by: Amelie Delaunay <amelie.delaunay@...s.st.com>
---
 drivers/dma/stm32-dma.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/dma/stm32-dma.c b/drivers/dma/stm32-dma.c
index 2283c500f4ce..83a37a6955a3 100644
--- a/drivers/dma/stm32-dma.c
+++ b/drivers/dma/stm32-dma.c
@@ -280,7 +280,7 @@ static enum dma_slave_buswidth stm32_dma_get_max_width(u32 buf_len,
 	       max_width > DMA_SLAVE_BUSWIDTH_1_BYTE)
 		max_width = max_width >> 1;
 
-	if (buf_addr % max_width)
+	if (buf_addr & (max_width - 1))
 		max_width = DMA_SLAVE_BUSWIDTH_1_BYTE;
 
 	return max_width;
@@ -757,7 +757,7 @@ static int stm32_dma_set_xfer_param(struct stm32_dma_chan *chan,
 		 * Set memory burst size - burst not possible if address is not aligned on
 		 * the address boundary equal to the size of the transfer
 		 */
-		if (buf_addr % buf_len)
+		if (buf_addr & (buf_len - 1))
 			src_maxburst = 1;
 		else
 			src_maxburst = STM32_DMA_MAX_BURST;
@@ -813,7 +813,7 @@ static int stm32_dma_set_xfer_param(struct stm32_dma_chan *chan,
 		 * Set memory burst size - burst not possible if address is not aligned on
 		 * the address boundary equal to the size of the transfer
 		 */
-		if (buf_addr % buf_len)
+		if (buf_addr & (buf_len - 1))
 			dst_maxburst = 1;
 		else
 			dst_maxburst = STM32_DMA_MAX_BURST;
-- 
2.25.1