Re: [PATCH 2/2] i2c: designware: enable High-speed mode for pcidrv
From: Jarkko Nikula
Date: Thu Oct 15 2015 - 04:33:29 EST
On 10/15/2015 08:46 AM, Xiang Wang wrote:
1. "bus speed mode" is a bit different from other parameters. Actually
it can be determined by the speed setting of "i2c devices" in ACPI
(I2CSerialBus). E.g. If i2c device uses 3MHz, we use High-speed mode
for this i2c bus.
2. If we hardcode speed setting in pci driver, we lose the
flexibility. A high-speed device may be connected to this i2c bus on
this board, but may be connected to another i2c bus on another board
design.
In this patch, we enumerate the i2c device in ACPI table to determine
the frequency setting. Then we set corresponding speed mode for this
i2c controller. The ACPI stuff is common for pci and plat driver. If
board design changes, we only change BIOS.
In conclusion, we have 2 solutions to set the i2c controller speed
mode (pci driver):
1) use hardcode value in pci driver
2) use frequency setting of "i2c device" in ACPI table (more flexible,
but looks a bit strange)
Do you have any preference/suggestions for above solutions? Thanks
I don't think we can hard code especially the high-speed mode because
most typically buses are populated with slower devices.
Things are a bit more clear when ACPI provides timing parameters for the
bus (for standard and fast speed modes at the moment in
i2c-designware-platdrv.c: dw_i2c_acpi_configure()) but still I think the
ACPI namespace walk may be needed against potential BIOS
misconfigurations. For instance if it provides timing parameters for all
speeds but there are devices with lower speed on the same bus.
I'd take these timing parameters as configuration data for bus features
but actual speed (speed bits in IC_CON register) is defined separately.
To me it looks only way to achieve that is to pick slowest device from
I2cSerialBus resource descriptors.
--
Jarkko
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/