Re: [PATCH v11 3/5] drivers/soc/litex: add LiteX SoC Controller driver

[Geert Uytterhoeven]

Hi Mateusz,

On Wed, Sep 23, 2020 at 12:10 PM Mateusz Holenko <mholenko@xxxxxxxxxxxx> wrote:
> From: Pawel Czarnecki <pczarnecki@xxxxxxxxxxxxxxxxxxxxxxxx>
>
> This commit adds driver for the FPGA-based LiteX SoC
> Controller from LiteX SoC builder.
>
> Co-developed-by: Mateusz Holenko <mholenko@xxxxxxxxxxxx>
> Signed-off-by: Mateusz Holenko <mholenko@xxxxxxxxxxxx>
> Signed-off-by: Pawel Czarnecki <pczarnecki@xxxxxxxxxxxxxxxxxxxxxxxx>

Thanks for your patch!

> --- /dev/null
> +++ b/drivers/soc/litex/Kconfig
> @@ -0,0 +1,15 @@
> +# SPDX-License_Identifier: GPL-2.0
> +
> +menu "Enable LiteX SoC Builder specific drivers"
> +
> +config LITEX_SOC_CONTROLLER
> +       tristate "Enable LiteX SoC Controller driver"
> +       depends on OF || COMPILE_TEST
> +       help
> +         This option enables the SoC Controller Driver which verifies
> +         LiteX CSR access and provides common litex_get_reg/litex_set_reg
> +         accessors.
> +         All drivers that use functions from litex.h must depend on
> +         LITEX_SOC_CONTROLLER.

I'm wondering if it makes sense to have them depend on a "simpler"
symbol instead, e.g. LITEX?

Currently the SoC controller is limited to I/O accessors and a simple
register compatibility check, but you may want to extend it with more
features later, so you probably want to keep the LITEX_SOC_CONTROLLER.
Hence you could add

    config LITEX
        bool

and let LITEX_SOC_CONTROLLER select LITEX.

> --- /dev/null
> +++ b/drivers/soc/litex/litex_soc_ctrl.c
> @@ -0,0 +1,194 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * LiteX SoC Controller Driver
> + *
> + * Copyright (C) 2020 Antmicro <www.antmicro.com>
> + *
> + */
> +
> +#include <linux/litex.h>
> +#include <linux/device.h>
> +#include <linux/errno.h>
> +#include <linux/of.h>
> +#include <linux/of_platform.h>
> +#include <linux/platform_device.h>
> +#include <linux/printk.h>
> +#include <linux/module.h>
> +#include <linux/errno.h>
> +#include <linux/io.h>
> +
> +/*
> + * The parameters below are true for LiteX SoC

SoCs

> + * configured for 8-bit CSR Bus, 32-bit aligned.
> + *
> + * Supporting other configurations will require
> + * extending the logic in this header.

This is no longer a header file.

> + */
> +#define LITEX_REG_SIZE             0x4
> +#define LITEX_SUBREG_SIZE          0x1
> +#define LITEX_SUBREG_SIZE_BIT      (LITEX_SUBREG_SIZE * 8)
> +
> +static DEFINE_SPINLOCK(csr_lock);
> +
> +/*
> + * LiteX SoC Generator, depending on the configuration,
> + * can split a single logical CSR (Control & Status Register)
> + * into a series of consecutive physical registers.
> + *
> + * For example, in the configuration with 8-bit CSR Bus,
> + * 32-bit aligned (the default one for 32-bit CPUs) a 32-bit
> + * logical CSR will be generated as four 32-bit physical registers,
> + * each one containing one byte of meaningful data.
> + *
> + * For details see: https://github.com/enjoy-digital/litex/wiki/CSR-Bus
> + *
> + * The purpose of `litex_set_reg`/`litex_get_reg` is to implement
> + * the logic of writing to/reading from the LiteX CSR in a single
> + * place that can be then reused by all LiteX drivers.
> + */
> +void litex_set_reg(void __iomem *reg, unsigned long reg_size,
> +                   unsigned long val)
> +{
> +       unsigned long shifted_data, shift, i;
> +       unsigned long flags;
> +
> +       spin_lock_irqsave(&csr_lock, flags);
> +
> +       for (i = 0; i < reg_size; ++i) {
> +               shift = ((reg_size - i - 1) * LITEX_SUBREG_SIZE_BIT);
> +               shifted_data = val >> shift;
> +
> +               writel((u32 __force)cpu_to_le32(shifted_data), reg + (LITEX_REG_SIZE * i));
> +       }
> +
> +       spin_unlock_irqrestore(&csr_lock, flags);
> +}
> +EXPORT_SYMBOL_GPL(litex_set_reg);

I'm still wondering about the overhead of loops and multiple accesses,
and the need for them (see also BenH's earlier comment).
If e.g. the register widths change for LiteUART (currently they're
hardcoded to one), would you still consider it using the same
programming interface, and thus compatible with "litex,liteuart"?

The spinlock access will probably become the source of lock contention
later, especially when considering SMP variants.

> +/*
> + * Check LiteX CSR read/write access
> + *
> + * This function reads and writes a scratch register in order
> + * to verify if CSR access works.
> + *
> + * In case any problems are detected, the driver should panic.
> + *
> + * Access to the LiteX CSR is, by design, done in CPU native
> + * endianness. The driver should not dynamically configure
> + * access functions when the endianness mismatch is detected.
> + * Such situation indicates problems in the soft SoC design
> + * and should be solved at the LiteX generator level,
> + * not in the software.
> + */
> +static int litex_check_csr_access(void __iomem *reg_addr)
> +{
> +       unsigned long reg;
> +
> +       reg = litex_get_reg(reg_addr + SCRATCH_REG_OFF, SCRATCH_REG_SIZE);
> +
> +       if (reg != SCRATCH_REG_VALUE) {
> +               panic("Scratch register read error! Expected: 0x%x but got: 0x%lx",
> +                       SCRATCH_REG_VALUE, reg);

Do you think the user will ever see this panic message? (see below)

> +               return -EINVAL;

Good ;-)  All of BUG()/WARN()/panic() may be compiled out, depending on
config options, so the system may continue running beyond the panic()
call.

> +static int litex_soc_ctrl_probe(struct platform_device *pdev)
> +{
> +       int result;
> +       struct device *dev;
> +       struct device_node *node;
> +       struct litex_soc_ctrl_device *soc_ctrl_dev;
> +
> +       dev = &pdev->dev;
> +       node = dev->of_node;
> +       if (!node)
> +               return -ENODEV;

FYI, this cannot happen.

> +
> +       soc_ctrl_dev = devm_kzalloc(dev, sizeof(*soc_ctrl_dev), GFP_KERNEL);
> +       if (!soc_ctrl_dev)
> +               return -ENOMEM;
> +
> +       soc_ctrl_dev->base = devm_platform_ioremap_resource(pdev, 0);
> +       if (IS_ERR(soc_ctrl_dev->base))
> +               return PTR_ERR(soc_ctrl_dev->base);
> +
> +       result = litex_check_csr_access(soc_ctrl_dev->base);
> +       if (result) {
> +               /* LiteX CSRs access is broken which means that
> +                * none of LiteX drivers will most probably
> +                * operate correctly
> +                */
> +               WARN(1, "Failed to validate CSR registers, the system is probably broken.\n");

WARN(result, ...)

But is this WARN() needed? You have already called panic() before.

> +       }
> +
> +       return result;
> +}
> +
> +static struct platform_driver litex_soc_ctrl_driver = {
> +       .driver = {
> +               .name = "litex-soc-controller",
> +               .of_match_table = of_match_ptr(litex_soc_ctrl_of_match)
> +       },
> +       .probe = litex_soc_ctrl_probe,
> +};
> +
> +module_platform_driver(litex_soc_ctrl_driver);

module_platform_driver() means this driver is probed quite late in the
boot sequence.  Currently the only other LiteX driver is liteuart, which
is probed at more or less the same time, but I can envision more early
drivers to be added later (typically interrupt/clock controllers and
timers not integrated into the main CPU core).
Note that even liteuart will run earlier, and thus access CSR registers
before the check has run, when using e.g. earlycon...

> --- /dev/null
> +++ b/include/linux/litex.h
> @@ -0,0 +1,24 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Common LiteX header providing
> + * helper functions for accessing CSRs.
> + *
> + * Implementation of the functions is provided by
> + * the LiteX SoC Controller driver.
> + *
> + * Copyright (C) 2019-2020 Antmicro <www.antmicro.com>
> + */
> +
> +#ifndef _LINUX_LITEX_H
> +#define _LINUX_LITEX_H
> +
> +#include <linux/io.h>
> +#include <linux/types.h>
> +#include <linux/compiler_types.h>
> +
> +void litex_set_reg(void __iomem *reg, unsigned long reg_sz, unsigned long val);
> +
> +unsigned long litex_get_reg(void __iomem *reg, unsigned long reg_sz);

Perhaps you can add static inline litex_{read,write}{8,16,32}() wrappers,
so drivers don't have to pass the reg_sz parameter explicitly,
and to make it look more like accessors of other bus types?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds