linux-kernel - Re: [PATCH v5 3/5] drivers/soc/litex: add LiteX SoC Controller driver

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Date:   Wed, 29 Apr 2020 13:11:54 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Mateusz Holenko <mholenko@...micro.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, devicetree@...r.kernel.org,
        linux-serial@...r.kernel.org
Cc:     Stafford Horne <shorne@...il.com>,
        Karol Gugala <kgugala@...micro.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Filip Kokosinski <fkokosinski@...micro.com>,
        Pawel Czarnecki <pczarnecki@...ernships.antmicro.com>,
        Joel Stanley <joel@....id.au>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Maxime Ripard <mripard@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Heiko Stuebner <heiko@...ech.de>,
        Sam Ravnborg <sam@...nborg.org>,
        Icenowy Zheng <icenowy@...c.io>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/5] drivers/soc/litex: add LiteX SoC Controller
 driver

On Sat, 2020-04-25 at 13:42 +0200, Mateusz Holenko wrote:
> From: Pawel Czarnecki <pczarnecki@...ernships.antmicro.com>
> 
> This commit adds driver for the FPGA-based LiteX SoC
> Controller from LiteX SoC builder.

Sorry for jumping in late, Joel only just pointed me to this :)

> + * 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;
> +
> +		__raw_writel(shifted_data, reg + (LITEX_REG_SIZE * i));
> +	}
> +
> +	spin_unlock_irqrestore(&csr_lock, flags);
> +}
> +
> +unsigned long litex_get_reg(void __iomem *reg, unsigned long reg_size)
> +{
> +	unsigned long shifted_data, shift, i;
> +	unsigned long result = 0;
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&csr_lock, flags);
> +
> +	for (i = 0; i < reg_size; ++i) {
> +		shifted_data = __raw_readl(reg + (LITEX_REG_SIZE * i));
> +
> +		shift = ((reg_size - i - 1) * LITEX_SUBREG_SIZE_BIT);
> +		result |= (shifted_data << shift);
> +	}
> +
> +	spin_unlock_irqrestore(&csr_lock, flags);
> +
> +	return result;
> +}

I really don't like the fact that the register sizes & sub sizes are
#defined. As your comment explains, this makes it harder to support
other configurations. This geometry should come from the device-tree
instead.

Also this while thing is rather gross (and the lock will not help
performance). Why can't CSRs be normally memory mapped always instead ?

Even when transporting them on a HW bus that's smaller, the HW bus
conversion should be able to do the break-down into a multi-breat
transfer rather than doing that in SW.

Or at least have a fast-path if the register size is no larger than the
sub size, so you can use a normal ioread32/iowrite32.

Also I wonder ... last I played with LiteX, it would re-generate the
register layout (including the bit layout inside registers potentially)
rather enthousiastically, making it pretty hard to have a fixed
register layout for use by a kernel driver. Was this addressed ?

Cheers,
Ben.