linux-kernel - Re: [PATCH v4 2/2] media: rc: ir-spi: constrain carrier frequency

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Message-ID: <94bc5863-f831-47b6-8bfd-57a807c8fe23@gmail.com>
Date: Thu, 12 Jun 2025 23:20:28 +0300
From: Cosmin Tanislav <demonsingur@...il.com>
To: Sean Young <sean@...s.org>
Cc: Mauro Carvalho Chehab <mchehab@...nel.org>, linux-media@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] media: rc: ir-spi: constrain carrier frequency

c

On 6/12/25 11:10 PM, Sean Young wrote:
> On Thu, Jun 12, 2025 at 09:02:59PM +0100, Sean Young wrote:
>> On Wed, Jun 11, 2025 at 11:35:21PM +0300, Cosmin Tanislav wrote:
>>> On 6/11/25 11:09 PM, Sean Young wrote:
>>>> On Wed, Jun 11, 2025 at 02:23:44PM +0300, Cosmin Tanislav wrote:
>>>>> Carrier frequency is currently unconstrained, allowing the SPI transfer
>>>>> to be allocated and filled only for it to be later rejected by the SPI
>>>>> controller since the frequency is too large.
>>>>>
>>>>> Add a check to constrain the carrier frequency inside
>>>>> ir_spi_set_tx_carrier().
>>>>>
>>>>> Also, move the number of bits per pulse to a macro since it is not used
>>>>> in multiple places.
>>>>>
>>>>> Signed-off-by: Cosmin Tanislav <demonsingur@...il.com>
>>>>> ---
>>>>>    drivers/media/rc/ir-spi.c | 6 +++++-
>>>>>    1 file changed, 5 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/media/rc/ir-spi.c b/drivers/media/rc/ir-spi.c
>>>>> index 50e30e2fae22..bf731204c81e 100644
>>>>> --- a/drivers/media/rc/ir-spi.c
>>>>> +++ b/drivers/media/rc/ir-spi.c
>>>>> @@ -21,6 +21,7 @@
>>>>>    #define IR_SPI_DRIVER_NAME		"ir-spi"
>>>>>    #define IR_SPI_DEFAULT_FREQUENCY	38000
>>>>> +#define IR_SPI_BITS_PER_PULSE		16
>>>>>    struct ir_spi_data {
>>>>>    	u32 freq;
>>>>> @@ -70,7 +71,7 @@ static int ir_spi_tx(struct rc_dev *dev, unsigned int *buffer, unsigned int coun
>>>>>    	memset(&xfer, 0, sizeof(xfer));
>>>>> -	xfer.speed_hz = idata->freq * 16;
>>>>> +	xfer.speed_hz = idata->freq * IR_SPI_BITS_PER_PULSE;
>>>>>    	xfer.len = len * sizeof(*tx_buf);
>>>>>    	xfer.tx_buf = tx_buf;
>>>>> @@ -98,6 +99,9 @@ static int ir_spi_set_tx_carrier(struct rc_dev *dev, u32 carrier)
>>>>>    	if (!carrier)
>>>>>    		return -EINVAL;
>>>>> +	if (carrier * IR_SPI_BITS_PER_PULSE > idata->spi->max_speed_hz)
>>>>> +		return -EINVAL;
>>>>
>>>> Just a nitpick.
>>>>
>>>> I think carrier * IR_SPI_BITS_PER_PULSE could overflow, and then the check
>>>> wouldn't work. It might be better to do:
>>>>
>>>> 	if (carrier > idata->spi->max_speed_hz / IR_SPI_BITS_PER_PULSE)
>>>>
>>>> However since IR_SPI_BITS_PER_PULSE is 16, which is just a shift left by 4,
>>>> I don't think this can be abused in any useful way.
>>>>
>>>
>>> I have another concern regarding overflow, inside ir_spi_tx().
>>>
>>> DIV_ROUND_CLOSEST() is called with buffer[i] * idata->freq and 1000000.
>>> buffer[i] comes from userspace, it's the number of microseconds for this
>>> pulse. It's unsigned int. lirc core already checks that each element
>>> is not bigger than 500000 microseconds. Issue is, at 500000, it would
>>> take a carrier frequency as low as 8590 to overflow the unsigned int.
>>
>> Interesting, you are right.
>>
>>> Maybe it would make sense to switch this one to mult_frac()? But we
>>> would lose rounding.
>>>
>>> mult_frac(buffer[i], idata->freq, 1000000)
>>>
>>> Optionally, we could cast buffer[i] to u64/unsigned long long, and use
>>> DIV_ROUND_CLOSEST_ULL.
>>>
>>> DIV_ROUND_CLOSEST_ULL((u64)buffer[i] * idata->freq, 1000000)
>>>
>>> Let me know what you think.
>>
>> I've given it some thought and I'm not sure there is a better solution. It's
>> an edge case of course, but we should deal with it correctly.
> 
> Actually could we use check_mul_overflow() for this?
> 

I think we're better off using DIV_ROUND_CLOSEST_ULL(), since after the
multiplication, there's a division by 1000000, which might bring us back
in 32-bit territory, even if the multiplication overflowed. If we use
check_mul_overflow(), we would just invalidate a case that would have
worked fine.

> Just an idea.
> 
> 
> Sean