From: Jason Wang <jasowang@redhat.com>
To: "Enrico Weigelt, metux IT consult" <lkml@metux.net>,
"Enrico Weigelt, metux IT consult" <info@metux.net>,
linux-kernel@vger.kernel.org
Cc: corbet@lwn.net, linus.walleij@linaro.org,
bgolaszewski@baylibre.com, mst@redhat.com,
linux-doc@vger.kernel.org, linux-gpio@vger.kernel.org,
virtualization@lists.linux-foundation.org,
linux-riscv@lists.infradead.org
Subject: Re: [PATCH v2 2/2] drivers: gpio: add virtio-gpio guest driver
Date: Tue, 8 Dec 2020 10:49:02 +0800 [thread overview]
Message-ID: <2882f118-3555-614c-33a0-30865673deb3@redhat.com> (raw)
In-Reply-To: <635faeb7-950e-e594-3217-69032ed9cbd1@metux.net>
On 2020/12/7 下午5:33, Enrico Weigelt, metux IT consult wrote:
> On 07.12.20 04:48, Jason Wang wrote:
>
> Hi,
>
>>>> Not a native speaker but event sounds like something driver read from
>>>> device. Looking at the below lists, most of them except for
>>>> VIRTIO_GPIO_EV_HOST_LEVEL looks more like a command.
>>> okay, shall I name it "message" ?
>>
>> It might be better.
> Okay, renamed to messages in v3.
>
>>>> #define VIRTIO_NET_OK 0
>>>> #define VIRTIO_NET_ERR 1
>>> hmm, so I'd need to define all the error codes that possibly could
>>> happen ?
>>
>> Yes, I think you need.
> Okay, going to do it in the next version.
>
>>>> If I read the code correctly, this expects there will be at most a
>>>> single type of event that can be processed at the same time. E.g can
>>>> upper layer want to read from different lines in parallel? If yes, we
>>>> need to deal with that.
>>> @Linus @Bartosz: can that happen or does gpio subsys already serialize
>>> requests ?
>>>
>>> Initially, I tried to protect it by spinlock (so, only one request may
>>> run at a time, other calls just wait until the first is finished), but
>>> it crashed when gpio cdev registration calls into the driver (fetches
>>> the status) while still in bootup.
>>>
>>> Don't recall the exact error anymore, but something like an
>>> inconsistency in the spinlock calls.
>>>
>>> Did I just use the wrong type of lock ?
>> I'm not sure since I am not familiar with GPIO. But a question is, if at
>> most one request is allowed, I'm not sure virtio is the best choice here
>> since we don't even need a queue(virtqueue) here.
> I guess, I should add locks to the gpio callback functions (where gpio
> subsys calls in). That way, requests are requests are strictly ordered.
> The locks didn't work in my previous attempts, but probably because I've
> missed to set the can_sleep flag (now fixed in v3).
>
> The gpio ops are already waiting for reply of the corresponding type, so
> the only bad thing that could happen is the same operation being called
> twice (when coming from different threads) and replies mixed up between
> first and second one. OTOH I don't see much problem w/ that. This can be
> fixed by adding a global lock.
>
>> I think it's still about whether or not we need allow a batch of
>> requests via a queue. Consider you've submitted two request A and B, and
>> if B is done first, current code won't work. This is because, the reply
>> is transported via rxq buffers not just reuse the txq buffer if I read
>> the code correctly.
> Meanwhile I've changed it to allocate a new rx buffer for the reply
> (done right before the request is sent), so everything should be
> processed in the order it had been sent. Assuming virtio keeps the
> order of the buffers in the queues.
Unfortunately, there's no guarantee that virtio will keep the order or
it needs to advertise VIRTIO_F_IN_ORDER. (see 2.6.9 in the virtio spec).
Btw, if possible, it's better to add a link to the userspace code here.
>
>>> Could you please give an example how bi-directional transmission within
>>> the same queue could look like ?
>> You can check how virtio-blk did this in:
>>
>> https://docs.oasis-open.org/virtio/virtio/v1.1/csprd01/virtio-v1.1-csprd01.html#x1-2500006
> hmm, still don't see how the code would actually look like. (in qemu as
> well as kernel). Just add the fetched inbuf as an outbuf (within the
> same queue) ?
Yes, virtio allows adding IN buffers after OUT buffer through descriptor
chaining.
Thanks
>
>>> Maybe add one new buffer per request and one new per received async
>>> signal ?
>> It would be safe to fill the whole rxq and do the refill e.g when half
>> of the queue is used.
> Okay, doing that now in v3: there's always at least one rx buffer,
> and requests as well as the intr receiver always add a new one.
> (they get removed on fetching, IMHO).
>
>
> --mtx
>
next prev parent reply other threads:[~2020-12-08 2:50 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-03 19:11 [PATCH v2 1/2] drivers: gpio: put virtual gpio device into their own submenu Enrico Weigelt, metux IT consult
2020-12-03 19:11 ` [PATCH v2 2/2] drivers: gpio: add virtio-gpio guest driver Enrico Weigelt, metux IT consult
2020-12-04 3:35 ` Jason Wang
2020-12-04 9:36 ` Enrico Weigelt, metux IT consult
2020-12-07 3:48 ` Jason Wang
2020-12-07 9:33 ` Enrico Weigelt, metux IT consult
2020-12-08 2:49 ` Jason Wang [this message]
2021-04-13 11:07 ` Alex Bennée
2020-12-05 7:59 ` Enrico Weigelt, metux IT consult
2020-12-05 19:32 ` Michael S. Tsirkin
2020-12-05 20:05 ` Enrico Weigelt, metux IT consult
2020-12-07 3:16 ` Jason Wang
2020-12-07 13:52 ` Michael S. Tsirkin
2020-12-07 20:34 ` Enrico Weigelt, metux IT consult
2020-12-07 3:12 ` Jason Wang
2020-12-07 13:53 ` Michael S. Tsirkin
2020-12-08 2:36 ` Jason Wang
2020-12-08 7:02 ` Enrico Weigelt, metux IT consult
2020-12-09 9:31 ` Jason Wang
2020-12-09 10:33 ` Enrico Weigelt, metux IT consult
2020-12-08 10:10 ` Michal Suchánek
2020-12-08 12:33 ` Enrico Weigelt, metux IT consult
2020-12-09 10:34 ` Michal Suchánek
2020-12-05 20:15 ` Howto listen to/handle gpio state changes ? " Enrico Weigelt, metux IT consult
2020-12-08 9:38 ` Linus Walleij
2020-12-08 14:04 ` Enrico Weigelt, metux IT consult
2020-12-08 16:15 ` Grygorii Strashko
2020-12-09 8:51 ` Linus Walleij
2020-12-09 11:19 ` Arnd Bergmann
2020-12-09 12:53 ` Linus Walleij
2020-12-09 20:22 ` Grygorii Strashko
2020-12-09 20:38 ` Arnd Bergmann
2020-12-10 13:32 ` Grygorii Strashko
2021-05-24 11:27 ` Viresh Kumar
2021-05-25 12:59 ` Enrico Weigelt, metux IT consult
2021-05-26 3:32 ` Viresh Kumar
2021-07-03 8:05 ` Michael S. Tsirkin
2021-07-05 3:51 ` Viresh Kumar
2020-12-07 9:55 ` [PATCH v2 1/2] drivers: gpio: put virtual gpio device into their own submenu Andy Shevchenko
2020-12-07 10:31 ` Bartosz Golaszewski
2020-12-07 11:22 ` Enrico Weigelt, metux IT consult
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