linux-kernel - Re: Could I (ab)use bus (struct bus

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Message-ID: <BANLkTimcR9Z54kgHDK63=jL1U-yvWspjRg@mail.gmail.com>
Date:	Tue, 19 Apr 2011 16:20:17 +0200
From:	Rafał Miłecki <zajec5@...il.com>
To:	Arend van Spriel <arend@...adcom.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	George Kashperko <george@...u.edu.ua>,
	Hauke Mehrtens <hauke@...ke-m.de>,
	Russell King <rmk@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Jonas Gorski <jonas.gorski@...il.com>,
	"b43-dev@...ts.infradead.org" <b43-dev@...ts.infradead.org>,
	Greg KH <greg@...ah.com>, Andy Botting <andy@...ybotting.com>,
	Larry Finger <Larry.Finger@...inger.net>
Subject: Re: Could I (ab)use bus (struct bus_type) for virtual Broadcom bus?

W dniu 19 kwietnia 2011 15:58 użytkownik Arend van Spriel
<arend@...adcom.com> napisał:
> On Sun, 17 Apr 2011 19:38:12 +0200, Arnd Bergmann <arnd@...db.de> wrote:
>
> Hi Rafał,
>
> As you probably expected I tried to integrate and use your bcmai/bcmaxi/?
> driver with our brcm80211 driver (making progress). In the mean time I tried
> to follow the discussions going on, but I am still catching up.
>
>> Compare this to the PCI bus type, which essentially deals with devices
>> that have a PCI configuration space that contains generic (irq, memory,
>> vendor/device ID, ...) registers along with device specific registers.
>
> How much alike is the (BCM)AXI bus type? My assumption was that each
> registered PCI device is handled by a single driver module. In the
> current(?) bcmai implementation each device driver is called with the
> appropriate device structure reference, but it will also have the bus
> structure reference and through that can also access other cores on the
> (bcm)axi bus. This seems to me a potential issue when there are no
> synchronization mechanisms in place (whether in a SoC configuration or
> PCI-hosted). Does the PCI bus type allow driver for device A access
> device B?

There are bus-specific infos that have to be available for drivers,
like board id, sprom, CC access. So bcmai(?) drivers have to have
access to bus struct.

One core driver should never touch other core, like 80211 touching ETH
or USB or anything. Exception is CC and maybe PCI(e).

I still have to make use of all windows to improve cores access.


>> A new bus_type really only makes sense if you expect a lot of devices
>> to use this and you want to have the probing in the bus. If you only
>> want to have a way to enumerate devices that get created by the
>> parent driver, you can also use platform devices.
>
> The main assumption of the (bcm)axi driver seems to be that each core can
> be considered as a device. Correct me if I am wrong, but I consider a
> device to be an entity providing a particular system function. So an
> ethernet device provides ethernet connectivity function, a mixer device
> provides sound mixing function, and so on. The cores within a chip are not
> always self-contained like this. To clarify let's say a system function is
> realized by programming core A, core B, and finally trigger core A to set
> the function in motion. This implies the need of coordination between the
> programming steps on those cores.

What cores may need to access each other? AFAIK so far we didn't meet
such a cores.

-- 
Rafał
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