linux-kernel - Re: [PATCH v3] arch/tile: add hypervisor-based character driver for SPI flash ROM

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Message-Id: <201106101841.27771.arnd@arndb.de>
Date:	Fri, 10 Jun 2011 18:41:27 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Chris Metcalf <cmetcalf@...era.com>
Cc:	linux-kernel@...r.kernel.org, Greg KH <gregkh@...e.de>,
	Eric Biederman <ebiederm@...stanetworks.com>,
	Chris Wright <chrisw@...s-sol.org>,
	Benjamin Thery <benjamin.thery@...l.net>,
	Phil Carmody <ext-phil.2.carmody@...ia.com>
Subject: Re: [PATCH v3] arch/tile: add hypervisor-based character driver for SPI flash ROM

On Thursday 02 June 2011, Chris Metcalf wrote:
> The first version of this patch proposed an arch/tile/drivers/ directory,
> but the consensus was that this was probably a poor choice for a place to
> group Tilera-specific drivers, and that in any case grouping by platform
> was discouraged, and grouping by function was preferred.
> 
> This version of the patch addresses various issues raised in the
> community, primarily the absence of sysfs integration.  The sysfs
> integration now handles passing information on sector size, page size,
> and total partition size to userspace as well.  In addition, we now
> use a single "struct cdev" to manage all the partition minor devices,
> and dynamically discover the correct number of partitions from the
> hypervisor rather than using a module_param with a default value.
> 
> This driver has no particular "peer" drivers it can be grouped with.
> It is sort of like an MTD driver for SPI ROM, but it doesn't group well
> with the other MTD devices since it relies on hypervisor virtualization
> to handle many of the irritating aspects of flash ROM management: sector
> awareness, background read for sub-sector writes, bit examination to
> determine whether a sector erase needs to be issued, etc.  It is in fact
> more like an EEPROM driver, but the hypervisor virtualization does require
> a "flush" command if you wish to commit a sector write prior to writing
> to a different sector, and this is sufficiently different from generic
> I2C/SPI EEPROMs that as a result it doesn't group well with them either.
> 
> The simple character device is already in use by a range of Tilera
> SPI ROM management tools, as well as by customers.  In addition, using
> the simple character device actually simplifies the userspace tools,
> since they don't need to manage sector erase, background read, etc.
> This both simplifies the code (since we can uniformly manage plain files
> and the SPI ROM) as well as makes the user code portable to non-Linux
> platforms that don't offer the same MTD ioctls.
> 
> Signed-off-by: Chris Metcalf <cmetcalf@...era.com>

Reviewed-by: Arnd Bergmann <arnd@...db.de>
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