Question about scsi_host_alloc

Question about scsi_host_alloc

[jidong xiao]

I see this function,struct Scsi_Host *scsi_host_alloc(struct
scsi_host_template *, int), I am wondering what does the second
parameter used for?Say,

/**
 * scsi_host_alloc - register a scsi host adapter instance.
 * @sht:	pointer to scsi host template
 * @privsize:	extra bytes to allocate for driver
 *
 * Note:
 * 	Allocate a new Scsi_Host and perform basic initialization.
 * 	The host is not published to the scsi midlayer until scsi_add_host
 * 	is called.
 *
 * Return value:
 * 	Pointer to a new Scsi_Host
 **/
struct Scsi_Host *scsi_host_alloc(struct scsi_host_template *sht, int privsize)

I mean the privsize, i.e. extra bytes, why do we need such extra
bytes?Who will use it?

Regards
Jason Xiao

Re: Question about scsi_host_alloc

[Matthew Wilcox]

On Wed, Jul 18, 2007 at 07:00:01PM +0800, jidong xiao wrote:
> I see this function,struct Scsi_Host *scsi_host_alloc(struct
> scsi_host_template *, int), I am wondering what does the second
> parameter used for?Say,
> 
> I mean the privsize, i.e. extra bytes, why do we need such extra
> bytes?Who will use it?

It's for the use of the driver.  Many drivers have a struct containing
additional information, such as:

struct scsi_qla_host {
        /* Linux adapter configuration data */
        struct Scsi_Host *host; /* pointer to host data */
        struct scsi_qla_host *next;
        struct device_reg __iomem *iobase;      /* Base Memory-mapped I/O addres
s */
[....]
        struct nvram nvram;
        int nvram_valid;
};

...
        struct scsi_qla_host *ha;
        host = scsi_host_alloc(&qla1280_driver_template, sizeof(*ha));
        ha = (struct scsi_qla_host *)host->hostdata;

It means the driver doesn't need to allocate the private data separately,
so it's a little more efficient.  You also don't need to worry about
reference counting it; it will be freed when the Scsi_Host is freed.

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