Re: [PATCH] rtc: remove VLA usage

[Gustavo A. R. Silva]

Hi Alexandre,

On 03/07/2018 03:25 PM, Alexandre Belloni wrote:
> On 07/03/2018 at 14:11:33 -0600, Gustavo A. R. Silva wrote:
> > In preparation to enabling -Wvla, remove VLA and replace it
> > with a fixed-length array instead.
> You should probably explain what VLA is and why this is important to do.

Sure. I can elaborate a little bit more.

> > Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
> > ---
> >   drivers/rtc/rtc-bq32k.c  | 2 +-
> >   drivers/rtc/rtc-mcp795.c | 2 +-
> >   2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/rtc/rtc-bq32k.c b/drivers/rtc/rtc-bq32k.c
> > index e8698e9..e4b234a 100644
> > --- a/drivers/rtc/rtc-bq32k.c
> > +++ b/drivers/rtc/rtc-bq32k.c
> > @@ -74,7 +74,7 @@ static int bq32k_read(struct device *dev, void *data, uint8_t off, uint8_t len)
> >   static int bq32k_write(struct device *dev, void *data, uint8_t off, uint8_t len)
> >   {
> >   	struct i2c_client *client = to_i2c_client(dev);
> > -	uint8_t buffer[len + 1];
> > +	uint8_t buffer[256];
> >   
> You chose to change len to 255, probably because this is a uint8_t but

Correct.

> this is way too much for this rtc, it only has 10 consecutive registers.

In that case probably the best solution is to add the following line to 
the module:

#define MAX_LEN 10

and update the rest of the code as follows:

uint8_t buffer[MAX_LEN + 1];


> >   	buffer[0] = off;
> >   	memcpy(&buffer[1], data, len);
> > diff --git a/drivers/rtc/rtc-mcp795.c b/drivers/rtc/rtc-mcp795.c
> > index 79e24ea..00e11c1 100644
> > --- a/drivers/rtc/rtc-mcp795.c
> > +++ b/drivers/rtc/rtc-mcp795.c
> > @@ -82,7 +82,7 @@ static int mcp795_rtcc_write(struct device *dev, u8 addr, u8 *data, u8 count)
> >   {
> >   	struct spi_device *spi = to_spi_device(dev);
> >   	int ret;
> > -	u8 tx[2 + count];
> > +	u8 tx[257];

For this particular case it seems to me that the following works just fine:

#define MAX_COUNT 7

u8 tx[MAX_COUNT + 2];

What do you think?

Thanks for your feedback.
--
Gustavo

> >   
> >   	tx[0] = MCP795_WRITE;
> >   	tx[1] = addr;
> > --
> > 2.7.4