[U-Boot] ARM: zynq: sdhci clock frequency init question

[Michal Simek <monstr@monstr.eu>]

Hi Krunal,

sorry for delay.

On 04/25/2014 06:19 PM, Krunal Desai wrote:
> Hi all -
> 
> I noticed that in zynq_sdhci.c, responsible for initializing PS SD controller(s), host controller max clock frequency is always set to 52MHz (http://git.denx.de/?p=u-boot.git;a=blob;f=drivers/mmc/zynq_sdhci.c;h=fdce2c2c10ec85c4a291532f927eae4a0b5627c9;hb=master#l34). In cases where user is using EMIO connectivity, max clock speed is limited to 25MHz. This results in out-of-spec operation as the divider calculation logic trusts the input to add_sdhci() and does not check the SLCR itself to confirm what the IO clock to SD controller actually is.
> 
> I think I saw OF/device-tree support patched in recently (I am on the 2013.4 tag); I think the "right" way to solve this is add capability in zynq_sdhci_init to read device-tree for 'clock-frequency' property, and use that to populate the arguments to add_sdhci() with that information, defaulting to a safe minimum (25MHz?) if no entry is found. Otherwise, I suppose a config could be added akin to 'ZYNQ_SD0_MIO'/'ZYNQ_SD1_MIO' to populate the correct minimum value.
> 
> Does this sound sane? I am thinking of implementing that as a patch for ourselves internally, but I think it will be of value to the greater community as well.

we didn't test this configuration that's why 52MHz is there as default case.
I think that should be easily possible to detect MIO setting
as we are doing for qspi/nand/usb.

We have this code in our xilinx repository and I have sent patches for mainline review
2 weeks ago or something like that.
I haven't got any NACK and I am going to send pull request to ARM custodian
when I fix fpga patches.

It means detection via MIO setting is reasonable way how to do it.
When you know that this go through EMIO 25MHz should be used.

Thanks,
Michal

-- 
Michal Simek, Ing. (M.Eng), OpenPGP -> KeyID: FE3D1F91
w: www.monstr.eu p: +42-0-721842854
Maintainer of Linux kernel - Microblaze cpu - http://www.monstr.eu/fdt/
Maintainer of Linux kernel - Xilinx Zynq ARM architecture
Microblaze U-BOOT custodian and responsible for u-boot arm zynq platform


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