On Tue, Feb 16, 2021 at 10:15 AM Jae Hyun Yoo
[off-list ref] wrote:

This driver uses byte mode that makes lots of interrupt calls
so it's not good for performance. Also, it makes the driver very
timing sensitive. To improve performance of the driver, this commit
modifies I2C node to support buffer mode which uses I2C SRAM buffer
instead of using a single byte buffer.

AST2400:
It has 2 KBytes (256 Bytes x 8 pages) of I2C SRAM buffer pool from
0x1e78a800 to 0x1e78afff that can be used for all busses with
buffer pool manipulation. To simplify implementation for supporting
both AST2400 and AST2500, it assigns each 128 Bytes per bus without
using buffer pool manipulation so total 1792 Bytes of I2C SRAM
buffer will be used.

AST2500:
It has 16 Bytes of individual I2C SRAM buffer per each bus and its
range is from 0x1e78a200 to 0x1e78a2df, so it doesn't have 'buffer
page selection' bit field in the Function control register, and
neither 'base address pointer' bit field in the Pool buffer control
register it has. To simplify implementation for supporting both
AST2400 and AST2500, it writes zeros on those register bit fields
but it's okay because it does nothing in AST2500.

AST2600:
It has 32 Bytes of individual I2C SRAM buffer per each bus and its
range is from 0x1e78ac00 to 0x1e78adff. Works just like AST2500
does.

See Documentation/devicetree/bindings/i2c/i2c-aspeed.txt for
enabling buffer mode details.

Signed-off-by: Jae Hyun Yoo <redacted>
Reviewed-by: Cédric Le Goater <clg@kaod.org>

Reviewed-by: Brendan Higgins <redacted>