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

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Message-ID: <52a1c253-0206-182e-5e41-03f94e7e7275@redhat.com>
Date:   Sun, 13 Mar 2022 09:07:17 -0700
From:   Tom Rix <trix@...hat.com>
To:     Noralf Trønnes <noralf@...nnes.org>,
        airlied@...ux.ie, daniel@...ll.ch
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/repaper: combine allocs in repaper_spi_transfer()


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

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;
>>   }
>>