linux-kernel - Re: [PATCH v18 6/6] ARM: socfpga: fpga bridge driver support

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Date:   Fri, 23 Sep 2016 16:13:19 +0200
From:   Steffen Trumtrar <s.trumtrar@...gutronix.de>
To:     Alan Tull <atull@...nsource.altera.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org,
        Moritz Fischer <moritz.fischer@...us.com>,
        Ian Campbell <ijc+devicetree@...lion.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        Matthew Gerlach <mgerlach@...era.com>,
        Dinh Nguyen <dinguyen@...nsource.altera.com>,
        delicious.quinoa@...il.com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v18 6/6] ARM: socfpga: fpga bridge driver support


Hi!

Alan Tull writes:

> Supports Altera SOCFPGA bridges:
>  * fpga2sdram
>  * fpga2hps
>  * hps2fpga
>  * lwhps2fpga
>
> Allows enabling/disabling the bridges through the FPGA
> Bridge Framework API functions.
>
> The fpga2sdram driver only supports enabling and disabling
> of the ports that been configured early on.  This is due to
> a hardware limitation where the read, write, and command
> ports on the fpga2sdram bridge can only be reconfigured
> while there are no transactions to the sdram, i.e. when
> running out of OCRAM before the kernel boots.
>
> Device tree property 'init-val' configures the driver to
> enable or disable the bridge during probe.  If the property
> does not exist, the driver will leave the bridge in its
> current state.
>
> Signed-off-by: Alan Tull <atull@...nsource.altera.com>
> Signed-off-by: Matthew Gerlach <mgerlach@...era.com>
> Signed-off-by: Dinh Nguyen <dinguyen@...nsource.altera.com>

(...)

> +static inline int _alt_fpga2sdram_enable_set(struct alt_fpga2sdram_data *priv,
> +					     bool enable)
> +{
> +	return regmap_update_bits(priv->sdrctl, ALT_SDR_CTL_FPGAPORTRST_OFST,
> +				  priv->mask, enable ? priv->mask : 0);
> +}

(...)

> +	/* Get f2s bridge configuration saved in handoff register */
> +	regmap_read(sysmgr, SYSMGR_ISWGRP_HANDOFF3, &priv->mask);
> +

Could you maybe add some documentation about this implicit information
shared between a bootloader and this driver?
I understand why you do this, but there must be a better way than
depending on something some bootloader wrote in some undocumented
register, no?
The documentation just says:

"These registers are used to store handoff infomation between the
preloader and the OS. These 8 registers can be used to store any
information. The contents of these registers have no impact on the
state of the HPS hardware"

If it is already agreed upon, that a bridge-enable property is okay,
why not add a port-enable property, too?

Regards,
Steffen Trumtrar

-- 
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