[PATCH 7/8] spi: mtk_snor: fix zeroed data in DMA read bounce path

[David Lechner]

On 4/28/26 10:54 AM, Julien Stephan wrote:
> Le lun. 6 avr. 2026 à 22:14, David Lechner <dlechner at baylibre.com> a écrit :
>>
>> From: Macpaul Lin <macpaul.lin at mediatek.com>
>>
>> Implement proper bounce buffer handling for the read path to fix zeroed
>> data when using DMA. In the bounce path, map the bounce buffer with
>> dma_map_single(), perform DMA using bounce_dma, then copy data from the
>> bounce buffer to the user buffer, and finally unmap with
>> dma_unmap_single().
>>
>> Signed-off-by: Macpaul Lin <macpaul.lin at mediatek.com>
>> Signed-off-by: David Lechner <dlechner at baylibre.com>
>> ---
>>  drivers/spi/mtk_snor.c | 17 ++++++++++++++---
>>  1 file changed, 14 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/spi/mtk_snor.c b/drivers/spi/mtk_snor.c
>> index 649bca5716c..21ac115e3c3 100644
>> --- a/drivers/spi/mtk_snor.c
>> +++ b/drivers/spi/mtk_snor.c
>> @@ -274,6 +274,7 @@ static int mtk_snor_read_bounce(struct mtk_snor_priv *priv,
>>  {
>>         unsigned int rdlen;
>>         int ret;
>> +       dma_addr_t bounce_dma;
>>
>>         if (op->data.nbytes & MTK_NOR_DMA_ALIGN_MASK)
>>                 rdlen = (op->data.nbytes + MTK_NOR_DMA_ALIGN) &
>> @@ -281,11 +282,21 @@ static int mtk_snor_read_bounce(struct mtk_snor_priv *priv,
>>         else
>>                 rdlen = op->data.nbytes;
>>
>> -       ret = mtk_snor_dma_exec(priv, op->addr.val, rdlen,
>> -                               (dma_addr_t)priv->buffer);
>> +       /* Map bounce buffer for DMA */
>> +       bounce_dma = dma_map_single(priv->buffer, rdlen, DMA_FROM_DEVICE);
>> +       if (dma_mapping_error(priv->dev, bounce_dma)) {this
> 
> nitpick: do we really need this?
> In u-boot dma_mapping_error is defined as a stub always returning 0...
> unless this patch is a direct port from a kernel patch?

It looks like all other callers of dma_mapping_error() in U-Boot are checking
the return value, so I think I will keep it in case the implementation ever
changes where it can return an error.

> 
> 
>> +               dev_err(priv->dev, "bounce buffer dma map failed\n");
>> +               return -EINVAL;
>> +       }
>>
>> -       if (!ret)
>> +       ret = mtk_snor_dma_exec(priv, op->addr.val, rdlen, bounce_dma);
>> +       /* Ensure DMA writes are visible to CPU and copy the requested bytes */
>> +       if (!ret) {
>> +               /* Synchronize cached data to CPU visible memory if needed */
>>                 memcpy(op->data.buf.in, priv->buffer, op->data.nbytes);
>> +       }
>> +       /* Unmap bounce buffer regardless of success/failure */
>> +       dma_unmap_single(bounce_dma, rdlen, DMA_FROM_DEVICE);
>>
>>         return ret;
>>  }
>>
>> --
>> 2.43.0
>>