linux-kernel - Re: [PATCH v7 19/19] staging: r8188eu: remove shared buffer for usb requests

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Date:   Fri, 17 Sep 2021 18:06:06 +0300
From:   Pavel Skripkin <paskripkin@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Larry Finger <Larry.Finger@...inger.net>,
        Phillip Potter <phil@...lpotter.co.uk>
Cc:     linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org,
        David Laight <david.Laight@...lab.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Subject: Re: [PATCH v7 19/19] staging: r8188eu: remove shared buffer for usb
 requests

On 9/17/21 18:03, Pavel Skripkin wrote:
> On 9/17/21 17:55, Greg Kroah-Hartman wrote:
>> On Fri, Sep 17, 2021 at 09:18:37AM +0200, Fabio M. De Francesco wrote:
>>> From: Pavel Skripkin <paskripkin@...il.com>
>>> 
>>> This driver used shared buffer for usb requests. It led to using
>>> mutexes, i.e no usb requests can be done in parallel.
>>> 
>>> USB requests can be fired in parallel since USB Core allows it. In
>>> order to allow them, remove usb_vendor_req_buf from dvobj_priv (since
>>> USB I/O is the only user of it) and remove also usb_vendor_req_mutex
>>> (since there is nothing to protect).
>> 
>> Ah, you are removing this buffer, nice!
>> 
>> But, just because the USB core allows multiple messages to be sent to a
>> device at the same time, does NOT mean that the device itself can handle
>> that sort of a thing.
>> 
>> Keeping that lock might be a good idea, until you can prove otherwise.
>> You never know, maybe there's never any contention at all for it because
>> these accesses are all done in a serial fashion and the lock
>> grab/release is instant.  But if that is not the case, you might really
>> get a device confused here by throwing multiple control messages at it
>> in ways that it is not set up to handle at all.
>> 
>> So please do not drop the lock.
>> 
>> More comments below.
>> 
> 
> We have tested this change. I've tested it in qemu with TP-Link
> TL-WN722N v2 / v3 [Realtek RTL8188EUS], and Fabio has tested it on his
> host for like a whole evening.
> 
> I agree, that our testing does not cover all possible cases and I can't
> say it was "good stress testing", so, I think, we need some comments
> from maintainers.
> 
> @Larry, @Phillip, does this change looks reasonable for this chip?
> 
			 ^^^^^^^^^^^

I mean mutex removal, sorry for confusion :)



With regards,
Pavel Skripkin