Re: [PATCH] drm/repaper: combine allocs in repaper_spi_transfer()

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On 3/13/22 8:41 AM, Noralf Trønnes wrote:

Den 13.03.2022 15.10, skrev trix@xxxxxxxxxx:
From: Tom Rix <trix@xxxxxxxxxx>

repaper_spi_transfer() allocates a single byte
for the spi header and then another buffer for
the payload.  Combine the allocs into a single
buffer with offsets.  To simplify the offsets
put the header after the payload.

Signed-off-by: Tom Rix <trix@xxxxxxxxxx>
---
  drivers/gpu/drm/tiny/repaper.c | 40 ++++++++++------------------------
  1 file changed, 12 insertions(+), 28 deletions(-)

diff --git a/drivers/gpu/drm/tiny/repaper.c b/drivers/gpu/drm/tiny/repaper.c
index 37b6bb90e46e1..22a6732f35a01 100644
--- a/drivers/gpu/drm/tiny/repaper.c
+++ b/drivers/gpu/drm/tiny/repaper.c
@@ -100,50 +100,34 @@ static inline struct repaper_epd *drm_to_epd(struct drm_device *drm)
  static int repaper_spi_transfer(struct spi_device *spi, u8 header,
  				const void *tx, void *rx, size_t len)
  {
-	void *txbuf = NULL, *rxbuf = NULL;
  	struct spi_transfer tr[2] = {};
-	u8 *headerbuf;
+	u8 *buf;
  	int ret;
- headerbuf = kmalloc(1, GFP_KERNEL);
-	if (!headerbuf)
+	buf = kmalloc(1 + len, GFP_KERNEL);
+	if (!buf)
  		return -ENOMEM;
- headerbuf[0] = header;
-	tr[0].tx_buf = headerbuf;
+	buf[len] = header;
+	tr[0].tx_buf = &buf[len];
I don't think this will work since the buffer is used directly for DMA
on some platforms[1] so the buffers need to be at the correct alignment
for that to work. For this reason I think it's better to leave this
as-is since we know the kmalloc buffers will always be useable for DMA
and the code is also easy to read and understand instead of calculating
offsets.

[1] tinydrm/mipi-dbi: Use dma-safe buffers for all SPI transfers
(a89bfc5d9a07)

Noralf.

  	tr[0].len = 1;
- /* Stack allocated tx? */
-	if (tx && len <= 32) {

How about a change to remove this ?

It seems like you are getting lucky.

reduce the to single txrx_buf

Tom

-		txbuf = kmemdup(tx, len, GFP_KERNEL);
-		if (!txbuf) {
-			ret = -ENOMEM;
-			goto out_free;
-		}
+	if (tx) {
+		memcpy(buf, tx, len);
+		tr[1].tx_buf = buf;
  	}
- if (rx) {
-		rxbuf = kmalloc(len, GFP_KERNEL);
-		if (!rxbuf) {
-			ret = -ENOMEM;
-			goto out_free;
-		}
-	}
+	if (rx)
+		tr[1].rx_buf = buf;
- tr[1].tx_buf = txbuf ? txbuf : tx;
-	tr[1].rx_buf = rxbuf;
  	tr[1].len = len;
ndelay(80);
  	ret = spi_sync_transfer(spi, tr, 2);
  	if (rx && !ret)
-		memcpy(rx, rxbuf, len);
-
-out_free:
-	kfree(headerbuf);
-	kfree(txbuf);
-	kfree(rxbuf);
+		memcpy(rx, buf, len);
+ kfree(buf);
  	return ret;
  }




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