linux-kernel - RE: [RFC PATCH net-next 7/9] ethtool: cmis_cdb: Add a layer for supporting CDB commands

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Date: Tue, 30 Jan 2024 07:55:32 +0000
From: Danielle Ratson <danieller@...dia.com>
To: Russell King <linux@...linux.org.uk>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "davem@...emloft.net"
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	<mlxsw@...dia.com>, Petr Machata <petrm@...dia.com>, Ido Schimmel
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Subject: RE: [RFC PATCH net-next 7/9] ethtool: cmis_cdb: Add a layer for
 supporting CDB commands

> > +int ethtool_cmis_page_init(struct ethtool_module_eeprom *page_data,
> > +			   u8 page, u32 offset, u32 length) {
> > +	page_data->page = page;
> > +	page_data->offset = offset;
> > +	page_data->length = length;
> > +	page_data->i2c_address = ETHTOOL_CMIS_CDB_PAGE_I2C_ADDR;
> > +	page_data->data = kmalloc(page_data->length, GFP_KERNEL);
> > +	if (!page_data->data)
> > +		return -ENOMEM;
> 
> Hmm, so every use is forced to use kmalloc() even when it's just one byte?
> That seems rather wasteful.
> 
> > +/* See section 9.4.1 "CMD 0040h: Module Features" in CMIS standard
> revision 5.2.
> > + * struct cmis_cdb_module_features_rpl is structured layout of the
> > +flat
> > + * array, ethtool_cmis_cdb_rpl::payload.
> > + */
> > +struct cmis_cdb_module_features_rpl {
> > +	u8	resv1[CMIS_CDB_MODULE_FEATURES_RESV_DATA];
> > +	__be16	max_completion_time;
> > +};
> 
> Does this structure need to be packed? I would suggest it does to ensure that
> the __be16 is correctly placed after the 34 bytes of u8.
> 
> Overall, I think the idea of always kmalloc()ing the data is a bad idea at the
> moment. We have no implementations that DMA to/from this buffer, and it
> means extra cycles spent, and an extra failure point each time we want to do a
> CMIS command.
> 
> It also introduces extra complexity, where we could just be passing a pointer
> to a function local variable or function local structure.
> 
> Unless we decide that the data pointer should be DMA-able from (in which
> case, that needs documenting as such) then I would suggest getting rid of the
> extra kmalloc()...kfree() bits.
> 
> Thanks.
> 

Will fix, thanks!

> --
> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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