linux-kernel - Re: [PATCH] drm/repaper: combine allocs in repaper_spi

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Date:   Sun, 13 Mar 2022 16:41:38 +0100
From:   Noralf Trønnes <noralf@...nnes.org>
To:     trix@...hat.com, airlied@...ux.ie, daniel@...ll.ch
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Noralf Trønnes <noralf@...nnes.org>
Subject: Re: [PATCH] drm/repaper: combine allocs in repaper_spi_transfer()



Den 13.03.2022 15.10, skrev trix@...hat.com:
> From: Tom Rix <trix@...hat.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 <trix@...hat.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) {
> -		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;
>  }
>