linux-kernel - Re: [PATCH v5 3/5] net: ax88796c: ASIX AX88796C SPI Ethernet Adapter Driver

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Date:   Wed, 04 Nov 2020 11:20:37 +0100
From:   Lukasz Stelmach <l.stelmach@...sung.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     jim.cromie@...il.com, Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Russell King <linux@...linux.org.uk>, netdev@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Bartłomiej Żolnierkiewicz 
        <b.zolnierkie@...sung.com>
Subject: Re: [PATCH v5 3/5] net: ax88796c: ASIX AX88796C SPI Ethernet
 Adapter Driver

It was <2020-11-04 śro 03:42>, when Andrew Lunn wrote:
>> +config SPI_AX88796C_COMPRESSION
>> +	bool "SPI transfer compression"
>> +	default n
>> +	depends on SPI_AX88796C
>> +	help
>> +	  Say Y here to enable SPI transfer compression. It saves up
>> +	  to 24 dummy cycles during each transfer which may noticably
>> +	  speed up short transfers. This sets the default value that is
>> +	  inherited by network interfecase during probe. It can be
>
> interface
>

Done.

>> +	  changed in run time via spi-compression ethtool tunable.
>
> changed _at_ run time...
>

Done.

>> +static int
>> +ax88796c_set_tunable(struct net_device *ndev, const struct ethtool_tunable *tuna,
>> +		     const void *data)
>> +{
>> +	struct ax88796c_device *ax_local = to_ax88796c_device(ndev);
>> +
>> +	switch (tuna->id) {
>> +	case ETHTOOL_SPI_COMPRESSION:
>> +		if (netif_running(ndev))
>> +			return -EBUSY;
>> +		ax_local->capabilities &= ~AX_CAP_COMP;
>> +		ax_local->capabilities |= *(u32 *)data ? AX_CAP_COMP : 0;
>
> You should probably validate here that data is 0 or 1. That is what
> ax88796c_get_tunable() will return.
>
> It seems like this controls two hardware bits:
>
> SPICR_RCEN | SPICR_QCEN
>
> Maybe at some point it would make sense to allow these bits to be set
> individually? If you never validate the tunable, you cannot make use
> of other values to control the bits individually.

Good point. What is your recommendation for the userland facing
interface, so that future changes will be least disruptive?

ax_local->capabilities |= ((*(u32 *)data) & SPICR_RCEN) ? AX_CAP_COMP : 0;

or rather

ax_local->capabilities |= ((*(u32 *)data) & (SPICR_RCEN | SPICR_QCEN)) ? AX_CAP_COMP : 0;

and possibly in the future (or now) split it into 

ax_local->capabilities |= ((*(u32 *)data) & SPICR_RCEN) ? AX_CAP_COMP_R : 0;
ax_local->capabilities |= ((*(u32 *)data) & SPICR_QCEN) ? AX_CAP_COMP_Q : 0;

(and appropriate masking abve and proper handling in
ax88796c_soft_reset()).

Kind regards
-- 
Łukasz Stelmach
Samsung R&D Institute Poland
Samsung Electronics

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