linux-kernel - Re: [PATCH] rtc: remove VLA usage

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Date:   Wed, 7 Mar 2018 17:14:02 -0600
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        linux-rtc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rtc: remove VLA usage



On 03/07/2018 05:12 PM, Alexandre Belloni wrote:
> On 07/03/2018 at 17:09:22 -0600, Gustavo A. R. Silva wrote:
>>
>> On 03/07/2018 05:01 PM, Alexandre Belloni wrote:
>>> 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@...eddedor.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.
>>>
>> I see...
>>
>> I was looking into this piece of code drivers/rtc/rtc-mcp795.c:302:
>>
>>          tmp[0] = (tmp[0] & 0x80) | bin2bcd(alm->time.tm_sec);
>>          tmp[1] = (tmp[1] & 0x80) | bin2bcd(alm->time.tm_min);
>>          tmp[2] = (tmp[2] & 0xE0) | bin2bcd(alm->time.tm_hour);
>>          tmp[3] = (tmp[3] & 0x80) | bin2bcd(alm->time.tm_wday + 1);
>>          /* set alarm match: seconds, minutes, hour, day, date and month */
>>          tmp[3] |= (MCP795_ALM0C2_BIT | MCP795_ALM0C1_BIT |
>> MCP795_ALM0C0_BIT);
>>          tmp[4] = (tmp[4] & 0xC0) | bin2bcd(alm->time.tm_mday);
>>          tmp[5] = (tmp[5] & 0xE0) | bin2bcd(alm->time.tm_mon + 1);
>>
>>          ret = mcp795_rtcc_write(dev, MCP795_REG_ALM0_SECONDS, tmp,
>> sizeof(tmp));
>>
> Yeah, the 128 or 255 bytes NVRAM is not yet supported but this is on my
> todo list
>

I got it.

Thanks
--
Gustavo