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 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;
> >   }
> >