Re: [PATCH v2 3/3] mmc: Add driver for LiteX's LiteSDCard interface

Geert Uytterhoeven
2021-12-07T14:16:37Z

Hi Gabriel,

On Tue, Dec 7, 2021 at 3:10 PM Gabriel L. Somlo <gsomlo@gmail.com> wrote:
> On Mon, Dec 06, 2021 at 01:24:49PM +0100, Geert Uytterhoeven wrote:
> > On Sat, Dec 4, 2021 at 9:41 PM Gabriel Somlo <gsomlo@gmail.com> wrote:
> > > LiteX (https://github.com/enjoy-digital/litex) is a SoC framework
> > > that targets FPGAs. LiteSDCard is a small footprint, configurable
> > > SDCard core commonly used in LiteX designs.
> > >
> > > The driver was first written in May 2020 and has been maintained
> > > cooperatively by the LiteX community. Thanks to all contributors!
> > >
> > > Co-developed-by: Kamil Rakoczy <krakoczy@antmicro.com>
> > > Signed-off-by: Kamil Rakoczy <krakoczy@antmicro.com>
> > > Co-developed-by: Maciej Dudek <mdudek@internships.antmicro.com>
> > > Signed-off-by: Maciej Dudek <mdudek@internships.antmicro.com>
> > > Co-developed-by: Paul Mackerras <paulus@ozlabs.org>
> > > Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
> > > Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>

> > > + host->clock = 0;
> > > + cpu = of_get_next_cpu_node(NULL);
> > > + ret = of_property_read_u32(cpu, "clock-frequency", &host->freq);
> > > + of_node_put(cpu);
> > > + if (ret) {
> > > + dev_err(&pdev->dev, "No \"clock-frequency\" property in DT\n");
> > > + goto err_free_host;
> > > + }
> >
> > This looks fragile.
> > Shouldn't the clock be obtained from a clock property in the mmc
> > device node, pointing to a clock provider?
> > How does the real clock tree look like?
>
> In a full LiteX SoC, the main sys_clock is used for cpu, buses, and as a
> input source for peripherals such as LiteSDCard (which then further
> subdivides it to obtain a 12.5--50.0 MHz sd_clock.
>
> But since we're considering supporting LiteSDCard as an independent IP
> block, the "source clock" frequency should indeed be specified as a DT
> property in the MMC device node. (I'll have to add that to the list of
> updates for litex_json2dts_linux.py as well, once we settle on what it
> will look like -- I'll try to make the change and corresponding update
> to the devicetree bindings doc for v3).
>
> LMK what you think.

Ideally there should be a "clocks" property with a phandle pointing to a
clock controller node (compatible with "litex,clk").

How does drivers/tty/serial/liteuart.c handle this? Oh, it doesn't ;-)

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds