linux-kernel - Re: [PATCH 0/7] mailbox: apple: peek

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Message-Id: <20220524145540.363553-1-jassisinghbrar@gmail.com>
Date:   Tue, 24 May 2022 09:55:40 -0500
From:   jassisinghbrar@...il.com
To:     Hector Martin <marcan@...can.st>
Cc:     Jassi Brar <jassisinghbrar@...il.com>,
        Anup Patel <anup.patel@...adcom.com>,
        Vinod Koul <vkoul@...nel.org>, Sven Peter <sven@...npeter.dev>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Mun Yew Tham <mun.yew.tham@...el.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Samuel Holland <samuel@...lland.org>,
        Michal Simek <michal.simek@...inx.com>,
        Arnd Bergmann <arnd@...db.de>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, dmaengine@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev
Subject: Re: [PATCH 0/7] mailbox: apple: peek_data cleanup and implementation

From: Jassi Brar <jassisinghbrar@...il.com>


Hi,

> The mailbox API has a `peek_data` operation. Its intent and
> documentation is rather ambiguous; at first glance and based on the
> name, it seems like it should only check for whether data is currently
> pending in the controller, without actually delivering it to the
> consumer. However, this interpretation is not useful for anything: the
> function can be called from atomic context, but without a way to
> actually *poll* for data from atomic context, there is no use in just
> checking for whether data is available.
>
Not exactly... the 'peek_data' is a means for client driver to hint the
controller driver that some data might have arrived (for controllers that
don't have anything like RX-Irq). The controller is then expected to dispatch
data after "not necessarily atomic" read.

  For example, a quick look at some bit may tell there is data available,
but actually reading the data from buffer may be non-atomic.
  In your case, you could already implement the patch-7/7 by simply calling it
peek_data() instead of poll_data(). Its ok to call mbox_chan_received_data()
from peek_data() because your data-read can be atomic.

Also some platforms may not have users of peek_data upstream (yet), so
simply weeding them out may not be right.

thanks.