linux-kernel - Re: [PATCH v6] i2c: virtio: add a virtio i2c frontend driver

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Date:   Thu, 11 Mar 2021 13:59:42 +0800
From:   Jie Deng <jie.deng@...el.com>
To:     Arnd Bergmann <arnd@...db.de>, Jason Wang <jasowang@...hat.com>
Cc:     Linux I2C <linux-i2c@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Wolfram Sang <wsa@...nel.org>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        conghui.chen@...el.com, kblaiech@...lanox.com,
        jarkko.nikula@...ux.intel.com,
        Sergey Semin <Sergey.Semin@...kalelectronics.ru>,
        Mike Rapoport <rppt@...nel.org>, loic.poulain@...aro.org,
        Tali Perry <tali.perry1@...il.com>,
        Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        yu1.wang@...el.com, shuo.a.liu@...el.com,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v6] i2c: virtio: add a virtio i2c frontend driver


On 2021/3/10 16:27, Arnd Bergmann wrote:
> On Wed, Mar 10, 2021 at 4:59 AM Jason Wang <jasowang@...hat.com> wrote:
>> On 2021/3/10 10:22 上午, Jie Deng wrote:
>>> On 2021/3/4 17:15, Jason Wang wrote:
>>>>
>>>>> +        }
>>>>> +
>>>>> +        if (msgs[i].flags & I2C_M_RD)
>>>>> +            memcpy(msgs[i].buf, req->buf, msgs[i].len);
>>>>
>>>> Sorry if I had asked this before but any rason not to use msg[i].buf
>>>> directly?
>>>>
>>>>
>>> The msg[i].buf is passed by the I2C core. I just noticed that these
>>> bufs are not
>>> always allocated by kmalloc. They may come from the stack, which may
>>> cause
>>> the check "sg_init_one -> sg_set_buf -> virt_addr_valid"  to fail.
>>> Therefore the
>>> msg[i].buf is not suitable for direct use here.
>> Right, stack is virtually mapped.
> Maybe there is (or should be) a way to let the i2c core code handle
> the bounce buffering in this case. This is surely not a problem that
> is unique to this driver, and I'm sure it has come up many times in
> the past.
>
> I see that there is a i2c_get_dma_safe_msg_buf() helper for this
> purpose, but it has to be called by the driver rather than the core,
> so the driver still needs to keep track of each address when it
> sends multiple i2c_msg at once, but maybe it can all be done
> inside the sg_table instead of yet another structure.
>
> At least this one avoids copying data that is marked with the
> I2C_M_DMA_SAFE flag.
>
>         Arnd

Make sense. Thanks Arnd. I will try to use those helper functions.

Regards,
Jie