RE: [PATCH v3 2/3] rtw88: enclose c2h cmd handle with mutex

[Tony Chuang <yhchuang@realtek.com>]

> I understand this is already queued up, but I still have a question:
> 
> On Wed, Jul 31, 2019 at 5:23 AM <yhchuang@realtek.com> wrote:
> > C2H commands that cannot be handled in IRQ context should
> > be protected by rtwdev->mutex. Because they might have a
> > sequece of hardware operations that does not want to be
> > interfered.
> 
> Can you elaborate on what interference you're looking at, exactly? I'm
> not a big fan of defensive addition of global locks, and this
> particular mutex isn't very targeted. It claims to be for mac80211
> callbacks, but you use it in quite a few places (some of which clearly
> don't make sense), and many of them are not related to mac80211
> callbacks AFAICT.

Basically it must protect rtwdev itself for mac80211 callbacks, but
work queue also should be protected, as work queue could be interfered
by mac80211 callbacks IIUC. And most of the time what I want to
protect is the "hardware operations". I might implicitly binds those
register read/write(s) with the rtwdev struct.

I think the problem here is I should give a better comment to better
describe the usage of the mutex. And I also want to keep it short.

> 
> To the contrary: this handler is called from the mac80211 work queue,
> which is ordered and therefore shouldn't be getting "interrupted"
> (e.g., conflicting commands). But then, you added the 'irqsafe'
> command, which gets run from the ISR...and doesn't hold this lock,
> obviously.

C2H work queue will not interrupt each other, but mac80211 callbacks
could, as most of the functions are consist of a sequence of hardware
register operations. And I don't want to use I/O under ISR context,
so here to split the C2H commands.

> 
> It may well be that you're correct here, but I'd like to see a better
> explanation for stuff like this. And maybe an update to the
> rtw_dev::mutex comments.
> 
> Brian
> 

Yan-Hsuan