linux-kernel - Re: [PATCH V15 3/6] i2c: tegra: fix maximum transfer size

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Date:   Fri, 8 Feb 2019 14:12:31 +0000
From:   Peter Rosin <peda@...ntia.se>
To:     Dmitry Osipenko <digetx@...il.com>,
        Sowjanya Komatineni <skomatineni@...dia.com>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "jonathanh@...dia.com" <jonathanh@...dia.com>,
        "mkarthik@...dia.com" <mkarthik@...dia.com>,
        "smohammed@...dia.com" <smohammed@...dia.com>,
        "talho@...dia.com" <talho@...dia.com>
CC:     "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>
Subject: Re: [PATCH V15 3/6] i2c: tegra: fix maximum transfer size

On 2019-02-08 13:46, Dmitry Osipenko wrote:
> 08.02.2019 1:25, Dmitry Osipenko пишет:
>>> 08.02.2019 1:16, Sowjanya Komatineni пишет:> 
>>>>> This is wrong, 65535 = 64 * 1024 - 1.
>>>>>
>>>>> Let's just use size-constants provided by kernel:
>>>>>
>>>>> static const struct i2c_adapter_quirks tegra_i2c_quirks = {
>>>>> 	.flags = I2C_AQ_NO_ZERO_LEN,
>>>>> 	.max_read_len = SZ_4K,
>>>>> 	.max_write_len = SZ_4K - I2C_PACKET_HEADER_SIZE, };
>>>>>
>>>>> static const struct i2c_adapter_quirks tegra194_i2c_quirks = {
>>>>> 	.flags = I2C_AQ_NO_ZERO_LEN,
>>>>> 	.max_read_len = SZ_64K,
>>>>> 	.max_write_len = SZ_64K - I2C_PACKET_HEADER_SIZE, };
>>>>>
>>>>>
>>>>> I'll take a look at other patches later today, no need to send out new version right now.
>>>>>
>>>>>
>>>>
>>>> SIZE_64K is 0x00010000 (65536)
>>>>
>>>> msg len is u16 and max that can be set to msg.len is 65535
>>
>> Ah, okay. Then it looks about right, maybe it's time to change u16 to u32 in the I2C core.
>>
> 
> Here is my suggestion:
> 
> 1) Use SZ_4K / SZ_64K that I suggested above.
> 
> 2) Squash this patch into the DMA-patch.
> 
> 3) Add another patch (before the DMA-patch) that changes max_read_len/max_write_len types to u32 in the include/linux/i2c.h

That is a bit naive and will not fly since the I2C core has a max message
size of 65535. Which is exposed to user space...

Given the above, saying .max_read_len = 65535 a NOP (and is in fact omitted
in places where it would be needed if the type is changed). I suggest that
it is omitted here as well. But SZ_64K - I2C_PACKET_HEADER_SIZE is still
needed of course. At least if I2C_PACKET_HEADER_SIZE is guaranteed to be
non-zero?

Cheers,
Peter