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To accommodate the different response time of SPI transfers on different boards and different SPI NOR devices, the Aspeed controllers provide a set of Read Timing Compensation registers to tune the timing delays depending on the frequency being used. The AST2600 SoC has one of these registers per device. On the AST2500 and AST2400 SoCs, the timing register is shared by all devices which is problematic to get good results other than for one device. The algorithm first reads a golden buffer at low speed and then performs reads with different clocks and delay cycle settings to find a breaking point. This selects a default good frequency for the CEx control register. The current settings are a bit optimistic as we pick the first delay giving good results. A safer approach would be to determine an interval and choose the middle value. Calibration is performed when the direct mapping for reads is created. Since the underlying spi-nor object needs to be initialized to create the spi_mem operation for direct mapping, we should be fine. Having a specific API would clarify the requirements though. Cc: Pratyush Yadav <p.yadav@ti.com> Reviewed-by: Joel Stanley <joel@jms.id.au> Tested-by: Joel Stanley <joel@jms.id.au> Tested-by: Tao Ren <rentao.bupt@gmail.com> Tested-by: Jae Hyun Yoo <quic_jaehyoo@quicinc.com> Signed-off-by: Cédric Le Goater <clg@kaod.org> Link: https://lore.kernel.org/r/20220509175616.1089346-9-clg@kaod.org Signed-off-by: Mark Brown <broonie@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.