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

Fri Jun 5 16:20:06 CEST 2020

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.

-- 
Tom
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