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SubjectRe: [PATCH] rtc: remove VLA usage
On 07/03/2018 at 16:39:51 -0600, Gustavo A. R. Silva wrote:
> 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@embeddedor.com>
> > > ---
> > > 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];
>

Seems better, yes.

>
> > > 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?
>

Nope, that RTC has a section of 255 bytes that could be read at once so
257 is the correct value.


--
Alexandre Belloni, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com

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