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

[Tom Rix]

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

Den 13.03.2022 15.10, skrev t...@redhat.com:

From: Tom Rix <t...@redhat.com>

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 <t...@redhat.com>
---
  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;
  }