Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
User-Agent: Cyrus-JMAP/3.5.0-alpha0-519-g27a961944e-fm-20210531.001-g27a96194
Mime-Version: 1.0
Message-Id: <55d28fea-c20e-4e60-8b5b-b5a03ccea0cf@www.fastmail.com>
In-Reply-To: <20210610100759.v57c5wa46ky33waj@ti.com>
References: <20210608040719.14431-1-git@johnthomson.fastmail.com.au>
 <20210610100759.v57c5wa46ky33waj@ti.com>
Date:   Fri, 11 Jun 2021 01:15:24 +0000
From:   "John Thomson" <git@johnthomson.fastmail.com.au>
To:     "Pratyush Yadav" <p.yadav@ti.com>
Cc:     "Miquel Raynal" <miquel.raynal@bootlin.com>,
        "Richard Weinberger" <richard@nod.at>,
        "Vignesh Raghavendra" <vigneshr@ti.com>,
        "Tudor Ambarus" <tudor.ambarus@microchip.com>,
        "Michael Walle" <michael@walle.cc>, linux-mtd@lists.infradead.org,
        linux-kernel@vger.kernel.org, "kernel test robot" <lkp@intel.com>,
        "Dan Carpenter" <dan.carpenter@oracle.com>
Subject: Re: [PATCH] mtd: spi-nor: write support for minor aligned partitions
Content-Type: text/plain
Precedence: bulk

On Thu, 10 Jun 2021, at 10:08, Pratyush Yadav wrote:
> On 08/06/21 02:07PM, John Thomson wrote:
> > Do not prevent writing to mtd partitions where a partition boundary sits
> > on a minor erasesize boundary.
> > This addresses a FIXME that has been present since the start of the
> > linux git history:
> > /* Doesn't start on a boundary of major erase size */
> > /* FIXME: Let it be writable if it is on a boundary of
> >  * _minor_ erase size though */
> > 
> > Allow a uniform erase region spi-nor device to be configured
> > to use the non-uniform erase regions code path for an erase with:
> > CONFIG_MTD_SPI_NOR_USE_VARIABLE_ERASE=y
> > 
> > On supporting hardware (SECT_4K: majority of current SPI-NOR device)
> > provide the facility for an erase to use the least number
> > of SPI-NOR operations, as well as access to 4K erase without
> > requiring CONFIG_MTD_SPI_NOR_USE_4K_SECTORS
> > 
> > Introduce erasesize_minor to the mtd struct,
> > the smallest erasesize supported by the device
> 
> Instead of having just a major and minor erase size, wouldn't it make 
> more sense to have a list of all supported erases on a sector? That is, 
> instead of hard coding two erase size instead of one, how about 
> generalizing the erase machinery to allow any number of erase sizes on a 
> sector and then choosing the most efficient one on run time?
> 
> For example, imagine a device can support 4K, 64K, and 256K erases on 
> each sector. Then you should be able to use 2 x 4K erases if you want to 
> erase 8K, 2 x 64K + 1 x 4K erases for 132K and so on.

Thank for you the feedback Pratyush,

Yes, this is what I am attempting to do.
mtdpart only sets if the partition can be writeable, it does not do the erasing.
Due to this, I thought that only the smallest erasesize should matter to mtdpart.
The erase is carried out by the mtd device, so for an SPI-NOR device,
spi_nor_erase_multi_sectors can be used to select and action the best combination of erases.
This is done by this patch when CONFIG_MTD_SPI_NOR_USE_VARIABLE_ERASE=y

Cheers,
-- 
  John Thomson