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Subject: Re: [PATCH] mailbox: bcm: Replace tasklet with threaded irq
From:   Rob Rice <rob.rice@broadcom.com>
In-Reply-To: <20210115035327.xckiigalv3qps7ke@offworld>
Date:   Fri, 15 Jan 2021 16:55:21 -0500
Cc:     Jassi Brar <jassisinghbrar@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Davidlohr Bueso <dbueso@suse.de>
Message-Id: <900BDAE9-8FF1-41C8-B909-D897BCAF3445@broadcom.com>
References: <20210115002134.117637-1-dave@stgolabs.net>
 <CABb+yY381GYcfAkLuzSTvNz2yHGs4Ap7Kgwiy9r0pXGGQt+ozg@mail.gmail.com>
 <20210115035327.xckiigalv3qps7ke@offworld>
To:     Davidlohr Bueso <dave@stgolabs.net>
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Davidlohr,

I haven=E2=80=99t worked on this driver for 4 years. So my memory is a litt=
le vague. But I have notes that we found using the tasklet was 40% faster t=
han threaded IRQ. We were trying to get performance up. So that seemed impo=
rtant at the time. I=E2=80=99m not prepared to defend that now. :-)

Rob

> On Jan 14, 2021, at 10:53 PM, Davidlohr Bueso <dave@stgolabs.net> wrote:
>=20
> On Thu, 14 Jan 2021, Jassi Brar wrote:
>=20
>> On Thu, Jan 14, 2021 at 6:21 PM Davidlohr Bueso <dave@stgolabs.net> wrot=
e:
>>>=20
>>> Tasklets have long been deprecated as being too heavy on the system
>>> by running in irq context - and this is not a performance critical
>>> path. If a higher priority process wants to run, it must wait for
>>> the tasklet to finish before doing so.
>>>=20
>>> Use a more suitable alternative such as threaded irqs and do the
>>> async work in process context.
>>>=20
>>> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
>>> ---
>> Please cc the author and other contributors to this file, esp when
>> this is vendor specific code.
>=20
> So looking at who to Cc I noticed:
>=20
> commit 8aef00f090bcbe5237c5a6628e7c000890267efe
> Author: Rob Rice <rob.rice@broadcom.com>
> Date:   Mon Nov 14 13:26:01 2016 -0500
>=20
>    mailbox: bcm-pdc: Convert from threaded IRQ to tasklet
>=20
>    Previously used threaded IRQs in the PDC driver to defer
>    processing the rx DMA ring after getting an rx done interrupt.
>    Instead, use a tasklet at normal priority for deferred processing.
>=20
> ... which is exactly the opposite of what modern Linux should be
> doing. Rob, could this not be done in process context?
>=20
> Thanks,
> Davidlohr


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