[PATCH 08/18] serial: add uart driver for MediaTek MT7620 SoC

[Weijie Gao <weijie.gao@mediatek.com>]

On Mon, 2020-10-26 at 22:51 -0600, Simon Glass wrote:
> Hi Weijie,
> 
> On Fri, 16 Oct 2020 at 01:36, Weijie Gao <weijie.gao@mediatek.com> wrote:
> >
> > This patch adds uart support for MediaTek MT7620 and earlier SoCs.
> >
> > The UART used by MT7620 is incompatible with the ns16550a driver.
> > All registers of this UART have different addresses. A special 16-bit
> > register for Divisor Latch is used to set the baudrate instead of the
> > original two 8-bit registers (DLL and DLM).
> >
> > The driver can be built without DM which is useful for tiny SPL.
> >
> > Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>
> > ---
> >  drivers/serial/Kconfig         |  20 ++
> >  drivers/serial/Makefile        |   1 +
> >  drivers/serial/serial.c        |   2 +
> >  drivers/serial/serial_mt7620.c | 350 +++++++++++++++++++++++++++++++++
> >  4 files changed, 373 insertions(+)
> >  create mode 100644 drivers/serial/serial_mt7620.c
> 
> Why do you need to build without DM? We have of-platdata which
> provides for smaller images. What is the SRAM size available?
> 

Actually it's the total size (spl+u-boot-lzma.img) that matters.
mt7620_rfb_defconfig is configured to be compatible with the original
u-boot from MTK SDK, and the maximum size is 192KiB.

Currently without SPL_DM, the final image u-boot-with-spl.bin is about
170KiB, the u-boot-spl.bin is only 20KiB.

If SPL_DM is enabled, the total size is 189KiB, which is very close to
the limit 192KiB, and the size of spl is almost doubled, 39KiB.
(Without DM, only serial_mt7620 is needed. However with DM, both
serial_mt7620, clk-mt7620 and misc are needed)

Considering that we may still add some feature to mt7620_rfb_defconfig
in the future, only 3KiB left is not enough. Building without DM is
apparently better.

I believe using of-platdata can only reduce few KiBs of SPL, because the
DM itself is the largest overhead.

> Regards,
> Simon