linux-kernel - Re: [PATCH 3/3] omap: add hwspinlock device

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Message-ID: <AANLkTikE1dCXW-7ScVoDgEK3sxai_TyFzstSG+zvNm2F@mail.gmail.com>
Date:	Wed, 20 Oct 2010 16:38:32 +0200
From:	Ohad Ben-Cohen <ohad@...ery.com>
To:	Kevin Hilman <khilman@...prootsystems.com>
Cc:	linux-omap@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, akpm@...ux-foundation.org,
	Greg KH <greg@...ah.com>, Tony Lindgren <tony@...mide.com>,
	Benoit Cousson <b-cousson@...com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Hari Kanigeri <h-kanigeri2@...com>, Suman Anna <s-anna@...com>,
	Simon Que <sque@...com>
Subject: Re: [PATCH 3/3] omap: add hwspinlock device

On Wed, Oct 20, 2010 at 1:53 AM, Kevin Hilman
<khilman@...prootsystems.com> wrote:
>> And to allow early board code to reserve specific hwspinlock numbers
>> for predefined use-cases, we probably want to be before arch_initcall.
>
> There's no reason for board code to have to do this at initcall time.

If we want to have allow both allocations of predefined hwspinlocks
with omap_hwspinlock_request_specific(int), and dynamic allocations
(where we don't care about the specific instance of the hwspinlock we
will get) with omap_hwspinlock_request(), we must ensure that the
former _specific() API will never be called after the latter.

If we will allow drivers to call omap_hwspinlock_request() before all
callers of omap_hwspinlock_request_specific() completed, then things
will break (because drivers might start getting hwspinlocks that are
predefined for dedicated use cases on the system).

So if we want the _specific API to work, we can only allow early board
code to use it in order to reserve those predefined hwspinlocks before
drivers get the chance to call omap_hwspinlock_request().

The tempting alternative is not to provide the
omap_hwspinlock_request_specific() API at all (which is something we
discussed internally).

Let's take the i2c-omap for example.

It sounds like it must have a predefined hwspinlock, but what if:

1. It will use omap_hwspinlock_request() to dynamically allocate a hwspinlock
2. Obviously, the hwspinlock id number must be communicated to the M3
BIOS, so the i2c-omap will publish that id using a small shared memory
entry that will be allocated for this sole purpose
3. we will make sure that 1+2 completes before the remote processor is
taken out of reset

This does not require any smart IPC and it will allow us to get rid of
the omap_hwspinlock_request_specific() API and its early-callers
requirement.

All we will be left to take care of is the order of the ->probe()
execution (assuming we want both the i2c and the hwspinlock drivers to
be device_initcall)

>
> This kind of thing needs to be done by platform_data function pointers,
> as is done for every other driver that needs platform-specific driver
> customization.

Why would we need platform-specific function pointers here ? I'm not
sure I'm following this one.

Thanks,
Ohad.

>
> Kevin
>
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