[PATCH] RFC: tiny-dm: Proposal for using driver model in SPL

Fri Jun 5 18:18:00 CEST 2020

Hi Tom,

On 5/6/20 11:20, Tom Rini wrote:
> On Thu, Jun 04, 2020 at 09:13:06PM -0600, Simon Glass wrote:
>> Hi Tom,
>>
>> On Tue, 2 Jun 2020 at 07:53, Tom Rini <trini at konsulko.com> wrote:
>>> On Mon, Jun 01, 2020 at 05:55:31PM -0300, Walter Lozano wrote:
>>>> Hi Simon,
>>>>
>>>> On 26/5/20 15:39, Walter Lozano wrote:
>>>>> Hi Simon,
>>>>>
>>>>> On 25/5/20 18:40, Simon Glass wrote:
>>>>>> Hi Tom,
>>>>>>
>>>>>> On Mon, 25 May 2020 at 14:57, Tom Rini <trini at konsulko.com> wrote:
>>>>>>> On Mon, May 25, 2020 at 02:34:20PM -0600, Simon Glass wrote:
>>>>>>>> Hi Tom,
>>>>>>>>
>>>>>>>> On Mon, 25 May 2020 at 13:47, Tom Rini <trini at konsulko.com> wrote:
>>>>>>>>> On Mon, May 25, 2020 at 09:35:44AM -0600, Simon Glass wrote:
>>>>>>>>>
>>>>>>>>>> This patch provides the documentation for a proposed
>>>>>>>>>> enhancement to driver
>>>>>>>>>> model to reduce overhead in SPL.
>>>>>>>>>>
>>>>>>>>>> The actual patches are not included here because
>>>>>>>>>> they are based on some
>>>>>>>>>> pending work by Walter Lozano which is not in final form.
>>>>>>>>>>
>>>>>>>>>> For now, the source tree is available at:
>>>>>>>>>>
>>>>>>>>>> https://gitlab.denx.de/u-boot/custodians/u-boot-dm/-/tree/dtoc-working
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Comments welcome!
>>>>>>>>> So here's my worry.  It's not clear, aside from the device tree, how
>>>>>>>>> much re-use of existing code we get with this.  It feels
>>>>>>>>> like it might
>>>>>>>>> be fairly minimal.  And at that point, are we not perhaps making too
>>>>>>>>> much work for ourselves compared with just excepting that there will
>>>>>>>>> need to be a place for non-abstracted-framework drivers?
>>>>>>>>> What do we do
>>>>>>>>> about TPL, when we get to the point of everything being
>>>>>>>>> converted to DM
>>>>>>>>> and as-needed tiny-DM but there's still TPL drivers?
>>>>>>>>> The reason we have
>>>>>>>>> SPL_FRAMEWORK as a symbol today is that we already had some
>>>>>>>>> SoCs/architectures (primarily PowerPC) that had "SPL" but it was very
>>>>>>>>> centric to the SoCs in question.
>>>>>>>>>
>>>>>>>>> The interface for example for mmc is:
>>>>>>>>> int spl_mmc_load_image(struct spl_image_info *spl_image, struct
>>>>>>>>> spl_boot_device *bootdev) and neither part of that is
>>>>>>>>> inherently DM.  So
>>>>>>>>> let it be MMC_TINY for the non-DM case and regular DM_MMC for the DM
>>>>>>>>> case.  I wonder if we could clean that up code a little
>>>>>>>>> if we let it be
>>>>>>>>> separate.
>>>>>>>>>
>>>>>>>>> The interface for example for spi is:
>>>>>>>>> int spl_spi_load_image(struct spl_image_info *spl_image,
>>>>>>>>> struct spl_boot_device *bootdev) and well, the same
>>>>>>>>> thing.  Or maybe we
>>>>>>>>> can even push that up to the spi_flash_load() call.
>>>>>>>>>
>>>>>>>>> But my worry is that a different set of abstractions here are still
>>>>>>>>> going to bring us in more overhead than writing drivers for the
>>>>>>>>> functionality we need directly, and if we define what's
>>>>>>>>> allowed in this
>>>>>>>>> limited case well, that might be good enough.
>>>>>>>> Some boards (e.g. x86) Need to read multiple things from the SPI flash
>>>>>>>> (such as FSP binaries), so I still think we will want a generic
>>>>>>>> reading interface.
>>>>>>>>
>>>>>>>> You could be right, but my hunch is that there is value in having
>>>>>>>> things more generic and the cost should be minimal. The value is that
>>>>>>>> hopefully writing a few C functions in the SPI driver will be enough
>>>>>>>> to enable tiny SPI on an SoC, reusing much of the code in the driver
>>>>>>>> (only the reading bits!). We won't need as much special-case code and
>>>>>>>> an entirely different way of configuring these devices for TPL/SPL.
>>>>>>>>
>>>>>>>> It has been interesting digging into the Zephyr model. It's drivers
>>>>>>>> are very basic and thus small. But there is still value in using the
>>>>>>>> device tree to assemble things.
>>>>>>>>
>>>>>>>> Anyway I'm not really sure at this point. It is just a hunch. I don't
>>>>>>>> think we can know all this until we have a bit more information.
>>>>>>>> Perhaps with a board with SPI, MMC and serial converted we would get a
>>>>>>>> better picture?
>>>>>>> I think it's absolutely the case that we'll have to convert something
>>>>>>> and see how it looks, then convert something else and see if it still
>>>>>>> looks good enough.  At a high enough level there's not really too much
>>>>>>> of a difference between what it sounds like you're proposing and what
>>>>>>> I'm proposing.  Possibly even in a progmatic way too.  We have (I think
>>>>>>> anyhow) fairly static board configurations in this case so we don't so
>>>>>>> much need to "probe" for possible drivers be told what our device
>>>>>>> hierarchy is and to initialize what we're going to use.
>>>>>> Yes, we may end up with special, separate code anyway, since if you
>>>>>> end up refactoring the driver so much (and putting tiny-dm tentacles
>>>>>> into it) that it becomes harder to maintain, it isn't a win.
>>>>>>
>>>>>> Basically I started out similar to what you are saying, with the idea
>>>>>> of just direct calls into the driver (e.g. the driver implements
>>>>>> serial_putc() and spi_read_flash()). But then I figured it is a very
>>>>>> small overhead to retain some sort of driver model, so I thought I'd
>>>>>> try that.
>>>>>>
>>>>>> I'll fiddle with this again in a week or so...
>>>>> Thanks for this proposal.
>>>>>
>>>>> I'm very interested in see where this implementation leads us, as I
>>>>> always felt that some work could be done in order to reduce the overhead
>>>>> of DM support in TPL/SPL. I'll review this work and hopefully come back
>>>>> to you with some comments.
>>>>>
>>>>> In the same sense, I feel that maybe we can add some additional
>>>>> intelligence to dtoc in order to produce a more customized code for
>>>>> TPL/SPL, maybe relaying in some custom stuff in u-boot.dtsi, but this is
>>>>> only a feeling.
>>>>>
>>>>>
>>>> I've been checking your work and found it very interesting. Let me share
>>>> some comments
>>>>
>>>>
>>>> I really like the trade-off between size and features in this implementation
>>>> and the way you get rid of not very useful data, such as strings and class
>>>> overhead.
>>>>
>>>>
>>>> I see that you parse drivers in order to build the relationship between
>>>> drivers and compatible strings among other things. I faced a similar
>>>> requirement in the series I proposed, however, I proposed the use of a
>>>> U_BOOT_DRIVER_ALIAS in order to explicitly declare the alias. Then main
>>>> reason behind this were to make code cleaner, avoid a lot of parsing and
>>>> having to take into account possible valid C code which is not cover by the
>>>> parsing.
>>>>
>>>> I have no problem with your approach, on the contrary I like the idea of
>>>> improving dtoc, but I think that if we take this path, we should put some
>>>> constrains and to document them to avoid unexpected behavior. Most of the
>>>> constrains maybe already used by all the drivers, like the way we declare
>>>> compatible strings, however any limitation in the parsing should be
>>>> documented.
>>>>
>>>>
>>>> The same goes for parsing config files which is also a nice improvement. I
>>>> feel that every step we give adding intelligence to dtoc is a step towards
>>>> footprint improvement.
>>> Would this allow us to make progress towards the idea of being able to
>>> discard compatibles (and referenced data) that won't be used at run time
>>> given the dts files for the boards we'll support at run time?
>> Do you mean just the compatible strings, or do you mean whole drivers?
> Compatible and data.  One of the "this is ugly but works" patches to
> reduce size on i.MX families was to guard compatible string + data (see
> drivers/mmc/fsl_esdhc_imx.c for example) with compile time checks.  If
> we could automate that somehow, that would be good.
>

Could you point to the "ugly but works" patch in order to have better 
context?

Regarding drivers/mmc/fsl_esdhc_imx.c I have been working in adding 
OF_PLATDATA support to it in order to reduce the SPL footprint on iMX6, 
which I think it could be a first step for further improvements.

https://patchwork.ozlabs.org/project/uboot/list/?series=167495&state=*

Regards,

Walter