Re: [PATCH 2/2] file-list: support special 'auto', 'block', 'nvmem' specifiers

[Sascha Hauer <sha@pengutronix.de>]

On Fri, Jun 09, 2023 at 09:37:56AM +0200, Ahmad Fatoum wrote:
> On 09.06.23 09:29, Sascha Hauer wrote:
> > On Thu, Jun 08, 2023 at 09:24:18AM +0200, Ahmad Fatoum wrote:
> >> Best practice is for each board to populate $global.system.partitions
> >> or $global.fastboot.partitions with a string exporting its flashable
> >> devices in a descriptive manner, e.g. "/dev/mmc0(eMMC),/dev/mmc1(SD)".
> >>
> >> This often goes into BSPs though, so upstream boards are left without
> >> default partitions, making use a bit cumbersome. Make this easier
> >> by providing three new magic specifiers:
> >>
> >>   - nvmem: exports all registered NVMEM devices (e.g. EEPROMs, Fuse banks)
> >>   - block: exports all registered block devices (e.g. eMMC and SD)
> >>   - auto:  currently equivalent to "nvmem,block". May be extended
> >>            to raw MTD and UBI in future
> >>
> >> This makes it easy to export devices on any board:
> >>
> >>   usbgadget -A auto -b
> >>
> >> or
> >>
> >>   usbgadget -S auto,/tmp/fitimage(fitimage)c
> >>
> >> Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
> >> ---
> >>  Documentation/user/usb.rst     | 17 +++++++++++++++++
> >>  common/block.c                 | 16 ++++++++++++++++
> >>  common/file-list.c             | 29 +++++++++++++++++++++++++++--
> >>  drivers/nvmem/core.c           | 16 ++++++++++++++++
> >>  include/block.h                |  6 ++++++
> >>  include/linux/nvmem-consumer.h |  8 ++++++++
> >>  6 files changed, 90 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/Documentation/user/usb.rst b/Documentation/user/usb.rst
> >> index f2f57ead98d4..6fed0f619b32 100644
> >> --- a/Documentation/user/usb.rst
> >> +++ b/Documentation/user/usb.rst
> >> @@ -73,6 +73,23 @@ Example:
> >>  
> >>    /dev/nand0.barebox.bb(barebox)sr,/kernel(kernel)rc
> >>  
> >> +Board code authors are encouraged to provide a default environment containing
> >> +partitions with descriptive names. For boards where this is not specified,
> >> +there exist a number of **partition** specifiers for automatically generating entries:
> >> +
> >> +* ``nvmem`` exports all registered NVMEM devices (e.g. EEPROMs, Fuse banks)
> > 
> > Blindly exporting NVMEM devices is not a good idea. As the description
> > says it's used for fuse banks. These are write-once and modifying them
> > is potentially dangerous. Also it's fastboot which means you can't even
> > read them before modifying them and there is no way to only partially
> > write them.
> > 
> > There might be good reasons to export some specific NVMEM device, but
> > then this should be explicitly exported and not by default.
> 
> The few fuse bank drivers that can actually blow fuses have a device
> variable that needs to be written to unlock write access.
> 
> But yes, the inability to write them partially makes the write
> useless. I will just resend without nvmem.

Side note: the fastboot protocol would even allow to do partial writes,
but I don't think there's a way to make use of them with the fastboot
tool.

Sascha


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