[PATCH] mtd: part: add generic parsing of linux,part-probe

[Hauke Mehrtens]

On 05/19/2015 10:25 AM, Jonas Gorski wrote:
> Hi,
> 
> On Tue, May 19, 2015 at 12:49 AM, Brian Norris
> <computersforpeace at gmail.com> wrote:
>> On Sun, May 17, 2015 at 07:35:12PM +0200, Hauke Mehrtens wrote:
>>> This moves the linux,part-probe device tree parsing code from
>>> physmap_of.c to mtdpart.c. Now all drivers can use this feature by just
>>> providing a reference to they device tree node in struct
>>
>> s/they/their/
>> or
>> s/they/the/
>> ?
>>
>>> mtd_part_parser_data.
>>>
>>> Signed-off-by: Hauke Mehrtens <hauke at hauke-m.de>
>>> ---
>>>  drivers/mtd/maps/physmap_of.c | 40 +---------------------------------------
> (snip)
>>>  drivers/mtd/mtdpart.c         | 38 ++++++++++++++++++++++++++++++++++++++
>>>  2 files changed, 39 insertions(+), 39 deletions(-)
>>>
>>> diff --git a/drivers/mtd/mtdpart.c b/drivers/mtd/mtdpart.c
>>> index cafdb88..b3059d6 100644
>>> --- a/drivers/mtd/mtdpart.c
>>> +++ b/drivers/mtd/mtdpart.c
>>> @@ -29,6 +29,7 @@
>>>  #include <linux/kmod.h>
>>>  #include <linux/mtd/mtd.h>
>>>  #include <linux/mtd/partitions.h>
>>> +#include <linux/of.h>
>>>  #include <linux/err.h>
>>>  #include <linux/kconfig.h>
>>>
>>> @@ -718,6 +719,35 @@ void deregister_mtd_parser(struct mtd_part_parser *p)
>>>  }
>>>  EXPORT_SYMBOL_GPL(deregister_mtd_parser);
>>>
>>> +static const char * const *of_get_probes(struct device_node *dp)
>>
>> Maybe a quick comment at the top to note that if non-NULL, its return
>> value must be kfree()'d by the caller? Could be helpful if we want to
>> move the code elsewhere. (I'm almost tempted to suggest moving this to
>> drivers/of/of_mtd.c, in fact.)
>>
>>> +{
>>> +     const char *cp;
>>> +     int cplen;
>>> +     unsigned int l;
>>> +     unsigned int count;
>>> +     const char **res;
>>> +
>>> +     cp = of_get_property(dp, "linux,part-probe", &cplen);
>>
>> I see this is not property is not documented in the first place. If
>> we're going to make it generically useful, we definitely need to
>> document it, including what the supported partition probe names are.
>>
>> I'm also kind of ambivalent on this point, but I might as well ask: does
>> the 'linux,...' naming make sense? We're not really supposed to be
>> putting pure software information in the device tree, let alone info
>> that's specific to Linux. I see how the partition parser could be (and
>> should be, really) considered OS-agnostic, though, and might deserve a
>> spot in the DT, at least if it's not safe to run arbitrary partition
>> parsers on all MTDs.
> 
> As long as we use the direct parser names, we should keep the linux prefix.
> 
> Actually a common issue is that many firmware partition tags aren't
> capable of describing the full flash layout, so partition parsers have
> to "cheat" and make a lot of assumptions, e.g. the location and the
> size of the bootloader. So wo would need to have a mix of static
> partitions (bootloader, bootloader config, calibration data) and
> dynamic ones (kernel, rootfs, overlay). So ideally we would pass the
> static partitions as device tree nodes, and let the partition parsers
> then be applied to partitions instead of the full mtd device. This
> would also work as a safeguard against mis-guessing important
> partitions like calibration data which aren't recoverable in case they
> get overwritten.
> 
> So my first Idea would be having something like this:
> 
> mtdevice at foo {
>         ...
>         bootloader: partition at 0 {
>                 reg = <0 0x10000>;
>                 device_type = "flash-partition";
>                 read-only;
>         };
> 
>         firmware: partition at 10000 {
>                 reg = <0x1000 0x7e0000>;
>                 device_type = "flash-partition";
>                 compatible = "brcm,trx";
>         };
> 
>         calibration: partition at 7f0000 {
>                 reg = < >;
>                 device_type = "flash-partition";
>                 read-only;
>         };
> };
> 
> Then the ofpart parsing could be integrated into the core, and
> partition parsers get run according to their compatibles on the
> individual partitions.
> 
> Of course this also might be total overengineering and completely the
> wrong place for devicetree as it's essentially configuration data, but
> since the now described partitions are now more or less "static", IMHO
> this would be a-okay.

This sounds like the problem Rafał wanted to solve in his patch "mtd:
add support for typed parsers splitting partitions"

This would also solve my problem and would make the bcm47xx part parser
a lot simpler and more secure for targets using device tree.
I still think we should add a alternative for the people that do not
want to rewrite their partition parser. Specifying the list of available
partition parser in the flash driver is a bad idea because it is the
wrong place.

I am in favor of both concepts, because there is already one driver
reading the partition parser list from device tree and it is implemented
pretty easy. Jonas Approach sounds nice because it really solves two
more problems.
Jonas approach would work on bcm47xx and bcm53xx targets, because there
the header is located after the boot loader and does not contain all
partitions. This could also be used to divide the partition containing
the kernel and the read only root fs in OpenWrt. Are there any more
users which could use this extended approach?

>>> @@ -754,6 +784,13 @@ int parse_mtd_partitions(struct mtd_info *master, const char *const *types,
>>>  {
>>>       struct mtd_part_parser *parser;
>>>       int ret = 0;
>>> +     const char *const *types_of = NULL;
>>> +
>>> +     if (data && data->of_node) {
>>> +             types_of = of_get_probes(data->of_node);
>>> +             if (types_of != NULL)
>>> +                     types = types_of;
>>> +     }
>>
>> I'm not sure I really like the semantics here. This means that the
>> device tree can override all Linux defaults. So we can't, for instance,
>> have the cmdline parser override all other selections (e.g., for
>> debugging), or make sure that 'ofpart' is available for all MTDs.
> 
> Letting the device tree override cmdline parsers is actually a valid
> use case for OpenWrt, where the bootloader passes a
> different/incompatible partition layout through mtdparts than what is
> actually used by the firmware on flash.
> 
> If we want to make it overrideable for debug purposes adding a
> commandline parameter like
> mtdparser="<mtdname>:<parser1[,parser2,...]>" might be a better
> option.
> 
>>>
>>>       if (!types)
>>>               types = default_mtd_part_types;
>>
>> In fact, I don't really like this existing hunk either ^^^. It has the
>> same effect, of killing things like cmdlinepart, which should be
>> available on all MTDs.
> 
> default_mtd_part_types contains cmdline part, so it should be
> available unless they pass their own parsers.
> 
> 
> Regards
> Jonas
>