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From:   Arnd Bergmann <arnd@kernel.org>
To:     Vinod Koul <vkoul@kernel.org>,
        Maxime Coquelin <mcoquelin.stm32@gmail.com>,
        Alexandre Torgue <alexandre.torgue@foss.st.com>,
        Amelie Delaunay <amelie.delaunay@foss.st.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Zhang Qilong <zhangqilong3@huawei.com>,
        dmaengine@vger.kernel.org,
        linux-stm32@st-md-mailman.stormreply.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: [PATCH] dmaengine: stm32-dma: avoid 64-bit division in stm32_dma_get_max_width
Date:   Tue, 19 Oct 2021 17:35:27 +0200
Message-Id: <20211019153532.366429-1-arnd@kernel.org>
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From: Arnd Bergmann <arnd@arndb.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.

In stm32_dma_set_xfer_param(), it is possible to pass a non-power-of-two
length, so this does not work. I assume this would in fact be a mistake,
and the hardware does not work correctly with a burst of e.g. 5 bytes
on a five-byte aligned address. Change this to only allow burst
transfers if the address is a multiple of the length, and that length
is a power-of-two number.

Fixes: b20fd5fa310c ("dmaengine: stm32-dma: fix stm32_dma_get_max_width")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 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..102278f7d13e 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 (!is_power_of_2(buf_len) || (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 (!is_power_of_2(buf_len) || (buf_addr & (buf_len -1)))
 			dst_maxburst = 1;
 		else
 			dst_maxburst = STM32_DMA_MAX_BURST;
-- 
2.29.2